I don't think anyone had an issue with Joe-the-private-citizen and his question. Obama answered his question and I think he answered it clearly.
It was the fact that John McCain elevated this Joe to the central theme of his campaign, mentioned him 20 times in the debate, referenced him in every speech, trotted him out at rallies and sent him out to talk to the networks. At which point Joe was not longer just a private citizen, but a spokesperson for the McCain campaign. Don't you think that before McCain's campaign decided to make this guy such a celebrity that they should have made sure their facts (and his) were straight?