Huh, learn something new every day. I had no idea Vimeo is older than YouTube, I've only really seen it around since they started offering HD.
I agree that it seems like a somewhat strange decision. It introduces an arbitrary judgement call - "this is a gaming video, this is machinima, this is somewhere in between but we'll remove it anyway", bitterness from their users, and lots of "this video was removed" links which will lead people to conclude that Vimeo is quite broken.
I can only conclude that the legal component of the issue is a lot more significant than it seems, or that the hardware costs are too high (in which case, how much revenue do "regular" videos bring?)
It's totally conceivable to me that people uploading and watching these gaming video cost more than they brought in for Vimeo.
It's part of the web mindset that more customers == more eyeballs == better. It's not always the case, especially if you're working with limited resources.
If cutting out the gamer videos helps Vimeo make money serving the niche they feel passionate about, more power to 'em.