Many covers can be posted on youtube. I have never understood the legality of doing that. Some get taken down, and others stay up.
What's legal and what you can get away with are not necessarily the same thing.
A lot of artists are content with putting a copyright claim on your cover song and taking the ad revenue so they can at least get a little something for compensation because going after some kid in his room with a lawsuit is generally counterproductive. Not only does it make no financial sense, it would be monumentally bad PR.
Janie Hendrix and Don Henley, on the other hand, would rather see Jimi's and the Eagles' legacies fade into obscurity and die so they takedown everything.