I've been seeing this used incorrectly in more and more places, including many supposedly professional outlets for writing.
In grade school, we learned a simple rule for keeping them straight:
"The principal is my pal." So, "principal" is for persons in a leading role.
"Principle is like a rule, both end in 'le.'" It's never a person.
I blame texting shortcuts for the decline in spelling, but I also blame spell-check. As long as the word matches a correct spelling, it goes through, even if it's the wrong one. Oh well...chalk this up to lost causes, I guess.