Vendors are extremely unagressive. Even if you're ready to attack to them, you can walk right their face and they won't attack you unless you attack them first or attempt to deal with them (don't attempt to deal with them unless you're at least dubious, although you have to be at least apprehensive to deal with them anyway). This is basically to keep them from clearing out creatures, like you said, and to keep them in their station most of the time to avoid disrupting business (you can't deal with a merchant if they're not in their stationed place, they'll just keep saying "I'm busy right now" until they return to their post).

Most merchants are about level 30, but they hit very hard for their level, and have a large number of hit points for their level. Any guard in the vicinity of an attacked merchant, and even other merchants, will quickly come to their aid. Even if you manage to kill one, merchants' corpses disappear immediately upon their death, and they give no experience regardless of what they conned for you. You'll take a big faction hit with their faction, as much as you would for killing a guard. So it's not worth it. Merchants aren't invincible, but they might as well be. The game is set up so there is no incentive to kill a merchant. At least killing guards gives you experience. Again, for the same reason they made merchants unagressive unless directly provoked, they made them very disadvantageous to try to kill, again to avoid disrupting business.