There is a subtle difference between games and music when it comes to copyright, and actually, I think the case is strong that MUSIC copyrights should have a 15-year life than game copyrights.
Fundamentally, the reason why copyrights exist...i.e. why the government creates a capacity for a very limited type of monopoly by protecting a certain kind of intellectual property...is to encourage additional production by creating a means for a profit from intellectual work. If there was no such thing as a copyright, then very few artists (musicians, writers, photographers, etc.) would actually create their art, unless they were independently wealthy and did it just for personal satisfaction, because creating art takes time and effort, and unless there's some kind of profit motive, many would be unwilling or financially unable to create the art. Similarly, few companies would be willing to fund the development of a game unless there's some way of earning profit from that game...and copyrights allow the company to earn the profit.
On the other hand, the public does have a certain right to "fair use" of material, and furthermore, by eliminating the copyright after a certain amount of time, you give a larger number of people a chance to enjoy the art and increase the total public good. As Quarto points out, it is (ideally) the public, acting through their agent of a representative government, that creates copyrights in the first place, because of its benefit to society.
So, in theory, there should be a copyright duration that maximally benefits the public interest...long enough so that there is sufficient profit motive to continue to produce art (including games), but short enough so that, once this profit motivation is satisfied, a maximal number of people have the opportunity to enjoy the art. Of course, as Quarto has pointed out, years of lobbying by folks such as Disney, the RIAA, et al., has caused copyright law to be structured to maximize profit, and not public good, by making copyright durations extremely long.
Here's where the subtle difference between games and music comes in. Music is, in theory, created by either a single individual or a small group of individuals, and these are the individuals that, ideally, should be compensated for the creation of their art, so that they keep producing and so that there is a profit motive for other talented musicians to produce the art. However, because of the way the music industry is structured (don't get me started, that's a lengthy discussion for another day), few musicians see a cent of royalties from their music after a certain number of years, and the only thing the copyright is doing there is allowing mega-giant corporations to make larger profits. For example, none of the surviving Beatles see any royalties from their music (I think Michael Jackson owned the copyrights of the Beatles collection...but I'm not sure who has it now). Reel Big Fish and REM are among bands who's former record companies continue to make profits from their songs, but who do not receive any royalties, because of bad contracts they were forced to sign when they were getting started. For a huge percentage of copyrighted song material, the original artist is actually dead, and the copyright is, at best, producing profit for that person's estate, but is probably just buying additional yachts for record industry CEO's. In other words, very long copyrights for music are not enriching the public good by encouraging more artists to make music, and hence do not serve a useful (to society) purpose.
Software, on the other hand, is generally more like patentable technology, where it requires a huge investment by a large corporation employing teams of programmers, art designers, writers, etc. The more long term profit can be earned, the more likely a company is to fund the creation of a new game. Hence, as long as there is any profitability for keeping a software copyright active, you are encouraging more companies to produce games and hence, theoretically, enhancing the public good.
So there's a much stronger case to be made for long software copyrights than long music copyrights.
In other words, I should have to still buy WC1, but I should be allowed to download 80's music for free.