My little opinion is as follows:
Everyone has pissy days. Everyone is an ass at some point to someone--it's unavoidable.
It's unfair to say, "well she/he is a _________, and should know better then to treat people like that."
For that blank spot insert: Waitress, Teacher, Doctor, Nurse, Secretary, Boss, Jehovahs Witness, Baptist, Born-again-Christian, Police Officer, Social Worker, Judge, Sales Clerk, etc.
I think we hold certain people to higher standandards, or people who hold themselves to higher standandards, are easy targets. It's not fair to judge a stranger by one experience you have had with them. We all feel entitled to bad days, and Everyone is entitled to a bad day.
Does that mean they are justified in their behavior, of course not. Does that mean it shouldn't be pointed out to them, of course not, tho tact and a cool head would be a must. You might even try buttering their ass with a "ahh having a bad day?" Just to shock 'em. Does that mean we sometimes don't look for things in those people to detract them--sometimes. Because of who they are, do we sometimes get just a little more annoyed with them--could be.
My whole point being--be curteous to everyone, and try not to judge people on one encounter.