I'm not defending Rodgers' actions, but it's a very different situation than Manning's. Peyton was one hit away from a career-ending neck injury and Luck was considered the best QB to come out of college since ... Peyton Manning. In fact, Peyton told the Colts they'd be stupid not to draft Luck.
Meanwhile, Rodgers is still healthy and productive, and the QB they drafted isn't expected to start anytime soon. Drafting his eventual replacement, like they did when they drafted Rodgers and still had Favre, was fine, but they should have at least gotten him some help in the receiving corps first. He has every right to be upset, but I agree with you that he shouldn't be airing it in public.