What Factors Have Led to the Duration of US Copyrights Continuously Increasing?

First of all, contrary to what TED stated, a Copyright IS a legal construct that ensures that an author or artist's original work is protected. A Copyright grants a creator of original work the exclusive rights to that work for a defined period of time, and it is the creator of that work who OWNS the Copyright. As a result, the Copyright owner has the right to receive compensation for any use of that original work. Furthermore, as a tangible property, it can be passed on as an inheritance, ensuring that the creator's descendants have the same legal protection.

Most countries, including the United States, follow the rules of the Berne Convention, in which that period of protection is defined as life plus 70 years. This means that once the creator dies, his descendants or heirs will be entitled to the same protection and compensation for 70 years after the creator's death. These heirs may now choose to sell that Copyright or grant licenses for the use of the work for certain compensation.

The duration of a Copyright has continuously changed to allow the creator and his descendants to benefit from those works for a longer period of time. As a published author myself, I take comfort in knowing that even after I die, my children and even grandchildren will continue to receive royalties from my works.

On the other hand, some entities have chosen to further protect their work by taking out a Trademark on a specific character, such as Mickey Mouse. In this case, the entity is staking claim to just that character, and the Trademark provides them legal protection against anyone using that individual character in any subsequent works. Even though the original "story" in which Mickey Mouse may no longer be protected by Copyright, the character itself IS protected by the Trademark. This is where you get into completely different types of licensing agreements that might include material products such as shirts and mugs.

In order to obtain such a Trademark and thereby restrict the use of a specific character, it must be demonstrated that the character is recognizable enough to make it truly unique. If someone wanted to obtain a Trademark for a minor character in a story that never really sold well in the first place, that request would be denied.

EDIT: When someone purchases a book by an author, the author receives royalties for that book. The transfer of royalties to the author is a direct result of the author granting permission for the book to be published in the first place. The author may also grant a percentage of the sales to go to the publisher, but the publisher only benefits because the author granted that permission in exchange for other services. The person who bought the book can do whatever they want with the physical copy of the book itself, including reselling it. However, the purchaser CANNOT write a new book using the same characters and setting from the original book without permission from the author. THIS is what is protected by the Copyright.

Regarding the discussion on whether or not a Copyright is tangible property, I found a document at the US Copyright office that provides further clarification on this matter:

A copyright may also be conveyed by operation of law and may be bequeathed by will or pass as personal property by the applicable laws of intestate succession.

Copyright is a personal property right, and it is subject to the various state laws and regulations that govern the ownership, inheritance, or transfer of personal property as well as terms of contracts or conduct of business.

To be rather boring:

• Primary accumulation and enclosures of the commons

Since the 1970s and social history, particularly reflecting on E. P. Thompson's work and the work of C.L.R. James, some of the strands of Marxist historiography reacting to the humanist Marxist turn in the 1940s and 1950s (Johnson-Forrest, the Hungarians, the Operismo) have taken to viewing the enforcement of property rights as far more contingent in any society than previous perhaps more "schematic" Marxist historians did. For an example lying outside of that tradition, and more inside the US new left tradition, there's a book on the floor describing medieval property law as a continuous contest between nascent bourgeoisie (in their true form as oppressed urban artisans) contra feudal extractions and dues.

Viewed from this angle, the historiography has bent towards viewing "primary accumulation" as much much more important, and as a continuous process occurring throughout the period of capitalism. This in some ways extends Lenin's thesis on Imperialism and the necessity of differential accumulation within capitalism to prop up the advanced metropole.

The effects of this is that Capitalism continuously seeks out new areas of life, society and human conduct to commodify, valorise and reduce to the value form. In the case of copyright and IP, this involves both the extension of the duration of property (with an obvious limit in effectively perpetual property), and the extension of the scope of property (gene patenting for example).

This is a relatively common understanding of the structure driving the extension of intellectual property in time amongst Marxists—though some cling to an older more schematic view and seek to deny the importance of recent enclosures.

Disney doesn't want the copyright over Micky Mouse to expire, so when the date is approaching, they "kindly ask" the government to extend it again.

Micky has been created in 1928, so the copyright duration will always be at least (current year - 1928 + 1).

Similarly, there has always been someone with similar interests and enough power to force this extension.