I have to disagree, as someone who's presently 6 weeks into a one-year postbac.
If my PD didn't believe I'd have a decent shot at med school (assuming of course that I do at least reasonably well here) I would rather not have been admitted. Likewise, if I start slipping or fail to improve or otherwise give the PD reason to think I'm not going to come out of postbac looking good, I want to hear about it far in advance of a stack of crappy grades.
A school that would tell a prospective applicant they may not have a good shot needs to deliver that message in a respectful and appropriate way, but the honesty part is something I actually think is pretty commendable. Like it or not, we're volunteering to be thrown into a selection process that is cutthroat and sometimes downright insane. I prefer all the early warning there is. I don't want warm fuzzies from my program, and it would make me nervous to get too many of them.
If they were simply rude, then yeah, screw 'em. 😉