Why do so many "legitimate" online videos use watermarked or clearly stolen from YouTube video clip?

Basically, why do seemingly legitimate websites or video content creators like Looper, Cracked, HuffingtonPost, College Humor and more, use clearly stolen videos from either YouTube or Torrents, instead of just getting them legitimately or at the very least not getting them from very poor video sources?

For example look at this college humor video

WARNING, VIDEO CLIP CONTAINS DONALD TRUMP

Look at how badly the video quality changes from clip to clip, and how some clips have watermarks while others are darkened to prevent YouTube copyright strikes.

Now, the obvious answer is “Because it’s the easiest/cheapest way” but I have two questions. Whats the legality of people stealing other Youtube videos for use in their own video, especially for “Big” sites like HuffingtonPost. Second, if they are stealing videos, why not just find a better piracy source? I’ve seen people use badly compressed or watermarked Simpsons clips in videos before, and do you know how easy it is to find clean DVD quality Simpsons clips online?