Queen Michael said:
And if you were to sing a song and people enjoyed it, and you knew that people would assume that the arrangement was yours unless you explained to them that it wasn't, then keeping quiet about the truth of the sing's origin would be an immoral thing to do since you're trying to make it seem like you created something you didn't create.
Nailhead, meet hammer, I think you'll get along fine.
That's exactly it, granted Glee have done a lot of catchy reworkings of classic songs, sometimes to their benefit (usually interesting mashups, a concept they certainly didn't create) and sometimes to their detriment (many o' these).
But they at least had the fact that they'd created said remix, whereas this is a straight up cover of someone else's work without their permission. I don't know the full legalities of it, and if they had a deal with Sir Mentadent and that overrides JC's rights to refusal then fair enough - but I imagine he'd be aware of that, and his continued chagrin suggests that's not the case.
If I've heard correctly, Glee don't approach many people whose songs they use, and after the event, most people don't care because it boosts their sales. However, on general principle, I'd be pissed if my work boosted their sales if I didn't approve of their format or product, even if it gave me a little gold to boot.
That Johnny C bit? Christ Glee, at least try and hide it a BIT.