The story so far: Jonathan Coulton wrote a great cover of baby got back, Glee ripped the cover off (everything from Coulton's melody to a note-for-note copy of the arrangement) without contacting or citing Coulton, and John Cheese wrote awesome satire of it.
Glee was wrong to not cite Coulton and give him a shout-out, but the idea that they robbed Coulton is stupid. The important thing to note here is that, if you think Glee "stole" the song from Coulton, needed to get permission from Coulton, or should owe money to Coulton (under just laws), what you are really saying is, "Sir-Mix-Alot should have to pay Johnathan Coulton when Glee covers Baby Got Back."
That's because "copyright" is another word for "monopoly." If you have a copyright on Baby Got Back, you have a complete monopoly over copies and most derivative works based on Baby Got Back. And if you have a monopoly, you charge monopoly prices, IE, "as much as the market will bear." So let's say Coulton comes along and covers your song, then Glee covers his cover. In one world Glee only has to pay you, the original artist, (that's this world, under our current laws). In another world, we say, "Coulton really poured a lot of creativity into that, and it is impressive. Glee not only has to pay you but also him if they cover his cover, to reward the creativity." Does this result in Glee paying more than they would in world one? No, because in both worlds there is a monopoly, in both worlds the rights to the song cost as much as the market will bear, and "as much as the market will bear" means just that. So if Glee isn't paying any more, how would they go about paying Coulton and Sir Mix-a-lot? Easy, Sir Mix-a-lot would have to get less.
If you think Glee ripped off Coulton (other than by being jerks and not citing him), you aren't mad that Glee took money from Jonathan Coulton, you are mad that Sir Mix-a-lot did. The issue here isn't whether the evil hacks at Glee should get to rob artists, it's whether one artist (Coulton) should get to take money from another (Sir Mixalot).