AI won't target gamelogics. There is I believe an invisible target addon that somebody made, but I've no idea where. (Search the forums, somebody might have posted a link.) The usual way is to hide a target - an M2 MG is popular - either underground or possibly behind a building. Or maybe get the AI to doTarget and doFire at the building itself, that might work.
To keep the players out of trouble use setCaptive true/false.