Thanks for your question.

Our sites fall under the DMCA Safe Harbor provisions. The content is user-submitted and user-moderated. User submissions are also often protected under the Fair Use doctrine in the United States as sufficiently original derivative works.

http://corp.cheezburger.com/legal/

The Cheezburger Network sites generate revenue through advertising, virtual currency, and physical goods (lolmart.com)

It seems like a lot of those sites focusing on funny pictures and videos of people getting hurt have tons of content that they don't own nor have the right to put online to earn a profit.

All they have to do is wait for the C&D and then take it down and not put it back up. In America at least, this is often a first step. Of course, if the value of a single particular image/video is so high that they could justify fighting it in court with a reasonable chance of winning they'd probably go down that road. I doubt there are many cases like that.

http://en.wikipedia.org/wiki/Cease_and_desist

1. Ads 2. What issues? I doubt ownership can be proven for any of it.

In one of my site I published an article about respiration and included an image from wikimedia. But mistakenly I chose a picture which is not in public domain. Within weeks somebody emailed me and made a big deal out of it. It was just one image still released under creative commons.

That's what I meant by issues when they do it in a large scale.