He was deemed a partial qualifier by the NCAA. I don't think it's a Pitt vs Xavier thing so much as a Conference thing.
Conferences such as the Big East and ACC do/did not accept partial qualifiers (I'm not sure if the BE still doesn't accept them, most non-P5 Conferences do which is what the BE is now).
"At least four of Cassell’s teammates at Notre Dame Prep took the classes in question and played their Division I basketball careers last year with no interruptions, the source said. Myles Davis, however, a freshman guard at Xavier, is in the same situation as Cassell. He has been ruled ineligible.
The source said Cassell did “everything that he was supposed to do in order to be eligible — not only at Maryland, but at any Division I school.”
Now, Cassell must find a new place to showcase his talents. He can transfer to a school that accepts partial qualifiers, redshirt this season and play in 2013-14. The ACC does not accept partial qualifiers."
LINK
Edit: I think the BE does allow partial qualifiers now. Marcus LoVett went to St. John's as a partial qualifier, the main reason most P5 schools eased up on him.