Simpley the way the market works, few people are going to buy a 5Dmk3 on a wim and go for a D800 instead purely because its not in stock, the same isnt true for entry level bodies.
Very good point, if average Joe gets excited about the new T4i goes to the store ready to buy a camera, if the T4i is not there, they just might pick-up some other companies latest camera and buy it.
The store doesn't care if they buy a Canon or not, they just want a sale.
Indeed, the wedding photographer looking to upgrade his 5D2 to a 5D3 on the other hand will just wait longer if he can't find it in stock, hence Canon don't run the risk of a production surplus.
