Are you sure you are not confusing local militia with an army? I've certainly had assassins camp out in enemy territories and *not* see any new armies coming in. There was however some local militia that made it tricky to gauge if I should attempt the province with the assassin or not.
I've been able to take provinces like that with assassins (and occasionally scouts as Jotun). So I don't really understand your complaint. The AI is not 'cheating' in the games I've played, at least not in this respect. You don't always get a fair measure of the local defense though, that's what makes things dicey from time to time. But realistically (and you know how much I hate to invoke that word
) does it make sense for your one assassin behind enemy lines to take an entire province of several thousand (potentially) inhabitants? No, obviously it doesn't, so lets keep realism out of this