TiVo people get mad when they call DIRECTV and the agents try to steer them away from these boxes but ultimtely it's because of this. After the TiVo wears off and features become important customers get upset.
I think there's a perception on DirecTV's part that they make more money by selling their own DVR than by selling a DirecTiVo, and _that_ is why they steer customers away from TiVos, not because of problems with the TiVo.
What pisses me off is DirecTV's dis-ingenuousness about TiVo. That and their constant insistence that they value my business while demonstrating near contempt at the value of my time.
As far as development yes TiVo is a vendor of DIRECTV but they handle the software development side of it. DIRECTV stamps it and sends it. It's not a blame game it's just who owns what and who is going to drive it.
I doubt TiVo is a 'vendor'. There's most likely a partnership agreement somewhere that spells out what each party has to do for the other.
Originally, DirecTV didn't know if consumers would buy a DVR, so they got someone else to fund development of a new 'technology'. TiVo happened to end up being the market leader for a while, and DirecTV happened to have a DVR product at a good time to be selling DVRs.
Eventually DirecTV (well, Rupert Murdoch) got tired of writing checks to TiVo and built their own, but with an arguably less elegant user interface. The agreement probably says that as long as DirecTV offers TiVos, they can also roll their own DVR, and TiVo can't sue for intellectual property infringement.
Then HD & MPEG4 came in, and TiVo's DirecTV wouldn't work any more. Of course, DirecTV could just wait until they all die in the field and say "so sorry, here's one of our _new_ DVRs," which probably would not have made them unhappy. So TiVo has a choice: build (software for) a new box that DirecTV can offer, or lose IP rights. DirecTV has a choice: offer TiVo's new box or be sued.
Result: a Mexican standoff that neither side is happy with or very motivated to put resources into resolving.
Of course, this is all speculation on my part.