The WWI Christmas truce of 1914 resulted in no consequences, in spite of clear warnings against such beforehand - why were there no courts-martial?

First, for clarity, it is worth noting the commanding officers of the British Expeditionary Force (BEF) in December 1914:

• Sir John French was the Commander-in-Chief of the BEF. Under him were
• Lieutenant-General Sir Douglas Haig, Commander BEF I Corps
• General Horace Lockwood Smith-Dorrien, Commander BEF II Corps. His chief of staff was
• Brigadier General G.T. Forrestier-Walker

The Christmas Truces and fraternization need to be looked at in a broader context. As indicated by the OP, fraternizations had been going on sometime before Christmas and it was these which led to the warnings from Brigadier General G.T. Forrestier-Walker and General Sir Horace Smith-Dorrien cited by the OP.

At this point, it is worth quoting a more complete version Smith-Dorrien's orders: (my highlighting)

Experience of this and every other war proves undoubtedly that troops in trenches in close proximity to the enemy slide very easily, if permitted to do so, into a “live and let live” theory of life. Understandings—amounting almost to unofficial armistices—grow up between our troops and the enemy, with a view to making life easier. …The attitude of our troops can be readily understood and to a certain extent commands sympathy.…Such an attitude is, however, most dangerous, for it discourages initiative in commanders and destroys the offensive spirit in all ranks.…Friendly intercourse with the enemy, unofficial armistices…and the exchange of tobacco and other comforts, however tempting and occasionally amusing they may be, are absolutely prohibited.

Smith-Dorrien had seen trench warfare in the Second Boer War (1899-1902) so he was speaking from long experience. Note that there is no specific mention of anyone being court-martialed at this point (5th of December). His orders are cited in full in Peter Hart's Fire and Movement: The British Expeditionary Force and the Campaign of 1914), after which the author comments:

It is interesting to note the understanding tone taken in this order: this was not the knee-jerk reaction of high-command of popular imagination.

Then, on 18th December 1914 at Ploegsteert Wood,

British and German troops agreed to permit one another to rise out of their trenches in order to bury their respective dead and collect the wounded. This was not particularly unusual, it had happened before

No action appears to have been taken over this truce; high command, if they even knew about it, probably considered it acceptable if no fraternization had taken place.

Then came the largely spontaneous Christmas truces, mostly starting on Christmas Eve and ending some 24 hours later (but going on longer in some areas). Fighting a war the troops may have been, but both German and British soldiers were sent Christmas packages, letters and even Christmas trees with the full knowledge (though not universal approval) of their respective military superiors. For the British,

There was even a special gift, commissioned for every soldier, originating from Princess Mary

"The Princess Mary Christmas gift box was decorated with an image of Mary and other military and imperial symbols and typically filled with an ounce of tobacco, a packet of cigarettes in a yellow monogrammed wrapper, a cigarette lighter, and a Christmas card and photograph from Princess Mary. Some contained sweets, chocolates, and lemon drops." Text & Image: Princess Mary Christmas gift box

Meanwhile, on the other side, one German officer commented with evident disdain on the treats organized for German soldiers:

This Christmas-gift stunt, organized by novelty-mongering, snobbish busybodies in a glare of publicity, creates such an unsavoury impression here that it fairly makes one sick. The fact that they make their appearance with a thousand packages of bad cigars, indifferent chocolate, and woollies of problematical usefulness, sitting in a car, seems to make them think they have a right to have the war shown to them like a leather factory.

Meanwhile, General Haig was writing his diary entry for 24th December:

Tomorrow being Xmas day, I ordered no reliefs to be carried out, and troops to be given as easy a time as possible.

Clearly, neither British nor German troops were expected to ignore Christmas. This may not have been approved of by all members of the respective High Commands, but these gifts were coming with the evident approval of their political and / or royal superiors.

Under such circumstances, the top officers may well have felt that court-martialing the thousands of men who took part in the Christmas truces was not a viable course of action. Also, there would have been the logistical problem of charging so many soldiers; as most of the truces seem to have been spontaneous, seeking out just the ring-leaders would have been difficult.

Another point to consider is that many truces started with a request by one side to remove and bury the dead lying in no man’s land. This then led to fraternization (exchange of cigarettes etc.) in many cases. Even detached and aloof senior officers would have realized that court-martialing soldiers under such circumstances would have damaged morale and possibly even led to mutinies.

And what of the C-in-C of the BEF? Sir John French, cited by Theresa Blom Crocker in her master’s thesis ‘A remarkable Instance’: The Christmas Truce and its Role in the Contemporaneous Narrative of the First World War (pdf):

wrote in 1919 that, although when the truce was first reported to him he “issued immediate orders to prevent any recurrence of such conduct,” he later realized that he had overreacted. In fact, French believed that, “had the question of the agreement upon an armistice for the day been submitted to me,” he might very well have agreed to it, as he always “attached the utmost importance to the maintenance of that chivalry in war which has almost invariably characterized every campaign of modern times in which this country has been engaged.”

Note here the emphasis was on preventing recurrences rather than punishing what had already happened.

On official responses reported in newspapers, there were none initially. Letters from soldiers were published alongside the usual official reports (or propaganda) from the front. Then, on 7th of January 1915, there was a report in The Times (my highlighting)

informing readers that, by Army order, “fraternizing and especially every approach to the enemy in the trenches, is forbidden, and in the future every infraction of this Order will be punished as treason”.

By mid January, the letters had dried up and it was almost as if the Christmas Truces had never happened. This was probably exactly what the British High Command wanted. In Germany, the Christmas truces were never even reported.

Sources:

The Famous Christmas Truce of 1914 (pdf)

The Christmas Truce of World War I

Michael Howard, The First World War (2002)

Combler les trous de la memoire

Fraternisations et trêve de Noël, décembre 1914-janvier 1915

First-hand Accounts of the Christmas Truce of 1914

WWI Christmas truce: haunting recollections of the short-lived peace