score:5
The UK Parliament at the time was run by the Tories, the Whigs being the opposition party. This meant that the war was essentially the Tories' baby. Meanwhile the opposition Whigs being were inclined to blame the entire situation on Tory misjudgment and incompetence. If the voting public came to believe the Whigs' view, then the Whigs might get to take over Parliament again.
The Tories' theory for how they would win the war was essentially that when the going got tough enough, the colonists would give up their silly grudge and come crawling back. So to them, a turncoat American general represented one small step for their strategy on the path to victory.
That not being a productive path for the opposition party, they'd be more inclined to treat him as a simple foreign traitor. So in effect, by going back to England Arnold made himself into a political football.
Upvote:2
The Whigs were left of center, and their sympathies were basically pro-American. Their great leader, William Pitt the Elder opined, "If I were an American, as I am an Englishman, while a foreign troop was landed in my country, I never would lay down my arms, never! never! never!" The Whigs would have been hostile to a "true blue" royalist, and the fact that Arnold's actions were treacherous only made matters worse in their eyes.
The Tories were right of center, and their sympathies were pro-Crown. Hence, they welcomed the actions of Benedict Arnold as "beneficial" to their cause, whatever distaste some may have had for him personally.