It's amazing that there are still people ignorant of copyright law.
1) There is plenty of case law demonstrating that websites ARE legally responsible if they do not remove copyright violations from servers they host upon notification.
2) A copyright violation is generally considered to be when a whole or significant part of a copyrighted work is used without license.
3) While it is true that you cannot protect inspirations under copyright, they are protected under other intellectual property laws if they are similar enough to create "reasonable confusion" (hence why Apple could legally eliminate Aqua-like skins if they so chose). Case law has put this under the term "Trade Dress".
Here's a link: http://www.tomwbell.com/writings/FullVTD.html#H2
Simply put, if you want to be a stickler, yes, you can stop people from creating "inspired" works of visual intellectual property.
4) Yes, even things that are "Free" are protected by intellectual property. If a website is found guilty of knowingly hosting copyrighted material, I believe under US law a plaintiff may sue for up to $125,000 (I'll have to dig more on that as it's been awhile). This law was primarily put into place so that small software developers (Back in the 80s) had some teeth when going after "pirate BBSes". Replace BBSes with websites and you have the same deal.
And finally:
Isn't it nearly always the case that those who don't create commodity level intellectual property that are the ones who seem to be really free with other people's?
So why hasn't Stardock gone after rippers directly?
Publicity. The best thing that could happen to one of these unethical sites would be to gain international media coverage and the quickest route to that would be for Stardock (or Microsoft or Apple or you name it) to go after them. So we'd only do such a thing as a last resort.