Maybe they like have no choice? Anyways I don't think a player will attack a ghoul on his own its usually a group doing it.
Usually they either have to be gone through to get somewhere (in or out), or are guarding (either intentionally or by location) something vital or outrageously expensive or powerful, and there is again no choice but to confront these things. I don't like the class/level system and I think the ghoul's level drain is a cheat.
Being that they can spread a disease that turns others into a ghoul (a.k.a. A plague), it may be the duty of the adventurer to neutralize the threat. Also thier surprise tactics are designed to make it so you have to fight them. If I were GM I would have a ghoul, or two, surprise attack paralyzing one target. Then have him dragged off, possibly underwater (good for a swamp scenario). While other ghouls block the PC's from helping their quickly dying friend. This leaves the PC's with little choice other than to fight to save their friend.