Exit only?

I don't get it. You've seen those doors that have the "exit only" signs right? They're above the door on the inside. How in the hell would people on the outside of that door know that it's exit only? I mean, would they even try to go in that door in the first place? What about us on the inside that see the sign...we are already inside so of course the only use we have for that door is to exit through it. Sure, there's the occasional rebel who tries to go in the out door (and vice versa)...but ha ha, the joke's on them because that door is locked on the outside.