What's stopped them is that it's complicated. Youporn and all the other porntube sites are a game of whac-a-mole as far as going after them. Youporn is biggest, yet still lives in shadows and grey area, much like roaches under a water heater. It's not a hard target like a warehouse full of knockoff DVD's in downtown L.A. that can be raided at dawn. It takes financial resources to employ the legal and technical people to even figure out what they're going after, after preliminary cease & desist letters sent to info@youporn.com only receive an message with "LOL" in reply. Consider that media giants like Viacom have a hard enough time getting stuff pulled from Youtube as it is, so porn companies getting stuff pulled from Youporn is near futile. Worse, porn is often dismissed as "unimportant" in terms of civil issues like copyright enforcement as far as law enforcement agencies are concerned. Further, many adult content providers are now uploading their own clips in attempt to drive traffic back to their sites, giving the Youporn owners ammunition to call it a "marketing tool" for adult websites, deflecting criticism with subterfuge.

I've never much bothered with youporn myself because it's a disjointed and chaotic mess of low-grade crap that the unwashed masses appear to be resonating with due to the "free is king" rule, but after a quick glance at their network it's clear they've spent time planning and digging in against any legal or criminal complaints that may come their way. They've got all their data mirrored in berlin, amsterdam and a few other offshores - places strategically chosen for laws (or lack thereof) friendly to their cause. If any part of their network got shut down in a particular country due to a court ruling, they wouldn't skip a beat.

That said, it's not unstoppable - though if they went down, all the other adult 'tube knockoffs are ready to pick up the slack. Ironically, the fear of 'getting too big' is what the owners seem to have feared from the beginning and is the reason they've wanted to sell it off and get out ASAP, since now it's become a giant hot potato where no legit company wants to pay millions to become the hard target for upset copyright holders to go after.

It's a tricky situation.