A TIE Pilot is seen ejecting in TESB actually, so it's canon that they CAN eject... He ejects on fire when an asteroid hits his fighter, but he ejects straight upward, nonetheless.
EU claims both Imperial and ALliance pilots CAN eject, but Imperials use an automated system as opposed to a manual one, and the Alliance can use either/or. Automated if they're prepared to use it (IE: set it to on, and have their life-supprt gear on), or manual if they need to get their stuff on if they know they're in a bad situation they aren't getting out of.
Consequently, it's one of the more fragile systems in all fighters, and often the first to be rendered inoperable it seems.... Then again, flight control was lost very often for me in the X-Wing sims as well. Hated when that happened. If I was lucky, nobody was targeting me and I either culd dump all power to engines and hope to outrun anyone who came for me (in a straight, unmaneuverable line away from combat), or I was hosed.... And if engines went too, I was more or less FUBAR.