I think it's just SOP that you don't let a guy transfer to a team on your schedule. Regardless of how good Kiel gets, he's been in the QB meetings for a year, he knows a lot about our program and it's not a good idea to let a guy like that play for a rival. Transfers between opponents do happen sometimes, for instance Amir Carlisle, but I think we are all still wondering exactly why Kiffin agreed to that.
I recall that Weis let a couple of our guys transfer to Stanford, too; Konrad Reuland, the FB/TE, and a kicker. I heard at the time that Reuland double-crossed Weis ... Weis said he'd grant the release if Reuland promised not to transfer to a team on our schedule, Reuland said he wouldn't, and then he did anyway. Not sure if that's true though. And the kicker may have been a walk on, don't know about him. Anyway, it does happen, but I don't think it's good practice. There are plenty of places Gunner can play without having to play for a team we play against.