I can actually answer this. The first answer is, it's required by the Americans with Disabilities Act, which is, in part, bureaucratic nonsense. But the more important answer is that it's all about independence. It's easy for the sighted to say "Well, why don't they just get help?" But that's not the point. Maybe a blind person's helper gets them to the machine, but it's psychologically important for them to be able hit their own buttons. As for the drive-up atm's, a lot of blind people take a cab to the atm and they wouldn't want to give the driver their PIN number.
Qualifications: I've designed accessible subway stations in Boston, and I've met with their ADA compliance guys, who are hardcore about this stuff.