It's called copyright law. If you remix a Lady Gaga song, it ain't fair use.
According to copyright.gov, fair use is determined by:
- The purpose and character of the use, including whether such use is of commercial nature or is for nonprofit educational purposes
- The nature of the copyrighted work
- The amount and substantiality of the portion used in relation to the copyrighted work as a whole
- The effect of the use upon the potential market for, or value of, the copyrighted work
It makes the difference if a Lady Gaga song is remixed to simply make a profit, or to promote awareness for a charity. Whether is it a long sample or a short snippet/interpolation, as well as if it competes with the original work if it
is for profit all need to be taken into account of as well.
"It ain't fair use" is a fairly brash thing to say because you fail to specify the remix's correlation (or lack thereof) to any of these pillars for fair use. You could be wrong or correct; but since you left this in the unknown it would be silly to draw this conclusion.
Copyright law is a pain either way; which is why it is more concisely decided by a judge than us.
With that said, networks occasionally play the role of the arbitrator when it comes to copyright because they do not want someone in their network who is bound to be a liability for them. Even though there are clauses in the contracts partners sign that make partners assume responsibility for their own content, they don't want to make money off of content that's infringing copyright because it makes the network look bad, especially to YouTube.
I wouldn't blame you for taking that stance, though, that remixes are not fair use. The video I linked is fairly recent and is supposed to replace the Happy Tree Friends one which didn't go into as much detail about fair use.
Hopefully you don't take my response as a disrespect to your authoritah; mods can get it wrong sometimes! Hopefully it's been an enlightening experience for you. As commentators, we often rely on fair use to publish and monetize our content, so it is always good to be familiar with the law of the land.