Can anyone say explain they think "Joe the Plumber" had any obligation to stick to the facts when asking Obama that question, and why failing to do so, either by accident or intentionally should leave him open to criticism?

As I've pointed out, he's not a politician, or a journalist, or a witness or a jury member in a court of law. There are a number of other situations in which there is indeed a duty to speak the truth about this kind of thing, but I can't see why asking a question to a politician should be one of those.

If he isn't actually a qualified plumber either, that is a matter purely between himself and his employers, or the people whose plumbing he may worked on.
.....................
"In your case, you can remove the word 'semi' and replace it with 'completely illiterate'

That would make it "completely illiterate-literate", Sanity. Moreover in that context "semi is not "a word", it's "a prefix" - "a verbal element placed at the beginning of a word to qualify meaning".