Given the political activism of such groups as AHF*, HIV is probably the last place third-party payers want to risk being seen as screwing patients. In other words, I agree with your post.
*They vociferously oppose everything GILD says and does (e.g. #msg-71169711).
I don't know but I think it depends on price, number of patients (with rare life-threatening diseases I'm certain insurers are reluctant to intervene), and profile of the generic vs the brand.