It's also mostly DirecTV's doing. They sell the unit as "DirecTV receiver with TiVo" - implying the "TiVo" functionality is more or less minor to the DirecTV service. From what it appears, it appears that TiVo has to cooperate with DirecTV at implementing DirecTV's orders.
If DirecTV says to TiVo, "I want you to disable ALL HACKS," TiVo will have to comply. It's about the only reasonable explaination on why the SA TiVos are still hackable while the DirecTiVos are slowly being locked down. If TiVo doesn't do it, DirecTV will probably just disable all DirecTiVo boxes, and say "Sorry, but all you TiVo users will have to go with Ultimate TV - we'll exchange your boxes for you". Or, basically, shaft TiVo (it's also probably why there isn't a DishTiVo).