Once upon a time, long ago, KO explained the ability of the Dark Knight and the Troll Mage to teach your commanders to "attack commander".
From my memory (so maybe not literal): "I guess he is a great tactician"
Doesn't sound like a bug to me. Maybe an unintended consequence, but what on earth *isn't* an unintended consequence in Dominions?
You are obviously free to make whatever house rules you wish, but to declare anyone who uses that trick as the equivalent of someone who hacks turn files is mighty high-handed.