Really, under which clause?

Is it for educational purposes? Using only a small portion of the original work? Is the creator deriving no commercial benefit?

(PS: Fair Use is a legal defense, not a positive rights clearance).

See also: Bridgeport Music Inc vs Dimension Films (https://en.wikipedia.org/wiki/Bridgeport_Music,_Inc._v._Dime...)

It's often at least the latter two things. Sampling is huge, though, without it we wouldn't have a lot of music. IANAL but there's clearly precedent.

So people are releasing these totally anonymously? Self-promotion is very much a commercial benefit.

Hah. That's taking things too far.

Most mashups I hear make heavy use of their component tracks, and I'm not even sure you could reasonably claim it as "sampling", much less fair use.