I've asked my dad, who is a tenured professor, about this, and he pretty much confirms what several people in this thread have said. He says it is not uncommon at all to informally use this. In fact, he often sees the phrase "non-tenure track professor" on job applications. This is at worst an oversight, and at best a thoughtless mistake.
(And you can't disagree with me because I know someone with a title!!!)
Well, not a professor. 
No, but there's a difference between a resume and a voter's guide list of credentials - who wrote them. As J. J. said, it makes staffer error very likely. He would have been widely referred to as a "professor of (whatever)." Unless Obama himself or someone who knows the distinction wrote it, an honest error is extremely likely.
