If the DM starts rocking attacks from 31ft in a case like this he's probably a terrible person and he should feel bad. It's bad enough when players get gamey like that, but it's worse when DMs do it. I am categorically not saying that monsters shouldn't have tactics and strategy, because they should, but targeting a character as described above is crap DMin IMO.. If a DM felt that strongly about the rule he should just disallowed it, not engage in petty revenge.