I hate how they lock out interiors after missions, the interior is still there, I can see it through cracks, so why bother locking it out?
But are you sure that the interrior has the same sort of quality to it?
I mean, if you look at the carshowroom where you begun doing missions with Franklin that interrior is also locked - but if you look at it from the outside it seems like the texture quality is not as good as when it's unlocked. Maybe it is just that interior. I don't know.
In GTA IV, however, that was not the case. The bank did for instance have a full, high-quality interior which you could glitch into. I don't see why they bothered to lock it.