Basicially its this simple do you really think video game companies are gonna chase down thousands and thousands of gfx sites that cut game characters out of gamewallpapers which most renders come from and try to shut them down. I highly doubt it since they would be throwing away money on lawyers for lawsuits and they would gain nothing from it. It is free advertising for them. Now sites like DA and CG its a single individual who's artwork you would be stealing from that sell their artwork for money unlike a video game company just does it for advertising.
Well, thats the tricky thing. Although the laws for copyrights proves what I said, the internet is different. There isn't an "Internet police" so most actions of "stealing" isn't punished. Also, the vast majority of people who would use another's works, doesn't publish it as their own, nor do they use it in a way that would benefit them.
