Posting any covers playing along to a copywritten song is a clear violation of federal law, and against Youtube's terms of service.
So many people get away with it because the record companies and such can't keep up with reporting the sheer number of videos. But many are often taken down as discovered.
People post disclaimers, but such disclaimers have no legal standing.
Now, posting your band playing a cover song that is completely 100% your bands performance is another issue. Now you're getting into music publishing area.
Technically, you're free to play a cover song as long as the publishing fee is paid.
As discussed in other threads, bars and clubs usually have a blanket license they pay for the rights to have music in their venues. Radio & TV stations generally pay per song.
This is a sticking point right now with Youtube.
A judge recently rules that Youtube owes ASCAP $1.6 million in back publishing fees
What has not been resolved is how this will work going into the future.
Side note: I'm reading the book "Bumping into Geniuses" by long time music executive Danny Goldberg (worked with Led Zepplin, Stevie Nicks, Nirvana, among others) and he discusses how most bands barely make any money selling records, but the bulk of a bands income comes from music publishing.