^^^well, he's threatened to for people putting stuff on youtube and all.
Since I know the full story surround that, I'll just say, that's quite the media-sensationalized overblown comment.
It's not like that at all.
Prince hired a company called Web Sheriff to protect his IP, including removals of his copyrighted content from unauthorized places.
Posting an artists work on any site without that artists permission is illegal, and he's hardly the only person to utilize the services of a company like that. They receive standard legal takedown notices just like anyone else, and just like any other copyright case, if you don't comply or continue to infringe, there is a possibility you will get sued. That's how the world works.
The thing was, Prince has his own members only site, lotusflow3r.com which is $77 a year subscription. One of the major features of the site is a video section that contains pretty rare videos. So, with that in mind you can see what the obvious conclusion is re: youtube.
I'm not saying I agree with it (although I am a member of his site and have enjoyed watching the rare content there) but he's Prince, he can (and will) do whatever the fuck he wants to do, but to trivialise it as "Prince sues his fans" is just inaccurate.