They control who can and can't enter the poop stall? :(
You're right to not reveal your disability (your privacy is important) but it's wrong if someone else who didn't need it was allowed to use the disabled stall.
Just curious though, how do you know the other guy didn't have a disability? As you know, sometimes it can be hard to tell.
That said if it actually was a case of prejudice then the employees boss should hear about it.