tube's legality?

[UpnCummer]

Hi, i've read a lot of threads and stuff where "illegal tube" pops out, the thing is that i don't understan what that means. Of course i'm not interested in getting my hands dirty in any illegal stuff, my question is about what makes a tube "illegal", is it because:

1) it has videos of bestiality, rape or -18 content?

2) is it because the site is a ripoff from another famous site-script?

3) Is it because it streams videos from companies or copyrighted videos(as all tubes do, as we all know)

I'm just trying to understand what makes a tube "illegal" and why some submitting sites like fucking1 will ban you if you have an embedded video from an illegal tube, for instance.