Do coaches know the rules?

Pretty standard match yesterday I was working as an AR in High School (which I like less and less by the day) until late in the second half. A throw in by white that was more like a corner, a laser. Anyway, it gets flicked and tips off of a defender and into the goal. 

Coach from team receiving the goal if I looked at the thrower. I said I did. Then why did I not flag the illegal throw he asks. Because it was not an illegal throw I tell him. So his assistant walks up to me and said, "You mean to tell me that was not an illegal throw?" Again, now more than slightly irritated that indeed, it was a legal throw. To top it off, one of the younger kids on the bench then says real loud "I am a referee and even I know that it was an illegal throw!". At that point I ask all of them, what part of the throw in was not legal. 

The coach, assistant coach and idiot sub all basically say that part of his foot was on the field. And so therein lies my problem. How can they be so ignorant of the rules? In my case, I did tell them that unless the foot is completely on the field, it is legal.

After the match, the coach had the gall to ask my the other two members of my team if indeed it was right what I was saying. Ah, what can I expect? A coach that knows the rules. Nah, why bother.