It's about principles,

[coffeeinbed]

not principals when talking about basic truths, laws and morals.
Not a principal.
Drives me bonkers....

[ReallyOverThis]

coffeeamazed nobody answered you! It is indeed really annoying. I am a lawyer and often dictate- if I forget to specify the ending on the tape the secretary will just pick one at random. I tried to explain once but she wasn't interested.

[cardibach]

I do a lot of musical theatre and I get irritated by the opposite! It's a principal role, not a principle one.

[chateauferret]

A "principal" (noun) is a Grade 7 official (or however they're graded nowadays) in the Civil Service. Otherwise it's an adjective.