What I don't get about this, if it is an "integrity" problem, is why they bring it up about that song NOW.
They're obviously NOT worried about other pop acts performing their songs since they let Kelly Clarkson sing a mash up of it at her concert that I saw last fall. Then again she also sang White Stripes and Patsy Cline as well. Maybe the difference is the whole selling it commercially afterward. So I don't think it's so much that the cast of Glee would do it worst or better than the original, just that they'd make money off it by selling their version.