The stupid (and counter intuitive) thing about this is that the person most likely to have violated copyright is not Glee but Coulton.
Coulton paid a compulsory license for his cover version, via the Harry Fox Agency. So Coulton has no protection under the Copyright Act.
Coulton agreed to;
Section 115 said:
not change the basic melody or fundamental character of the work, and shall not be subject to protection as a derivative work under this title, except with the express consent of the copyright owner
Also, in theory, Sir Mix A Lot (the original artist) could sue Coulton claiming that Coulton's version is not a direct copy and so is not covered by the statutory licience Coulton purchased.
That is how fubar'ed the copyright system has got....