What a teacher/principle does in their free time they are free to do. Read up on Pickering Vs. The Board of Education.
I disagree that Pickering v BOE provides a blanket statement that whatever a teacher or principal does on his free time cannot result in termination.
School employees who engage in illegal behavior outside of school are certainly subject to termination.
Part of the decision in Pickering was based on the belief that Pickering's letter did not affect the best interests of the school, nor was it "detrimental to the efficient operation and administration of the schools of the district" as the Board stated. If the US Supreme Court had found otherwise, I believe the decision would have been different.
Teachers are free to express their opinions--and yes they blog too ..... Really, what someone does on their free time is merely their choice. They are free to give their own opinions as a regular citizen of your county rather than that of Principle.
If you read the footnotes of the decision, I think you'll find that this simply isn't always true. Teachers and principals can, and have been, dismissed as a result of something that was said or done on their free time. If it affects the operation of the school or the employee's role in the school, then the termination will probably be upheld.
The principal's comment about parents (as we lovingly call all the parents ***holes) is one of those actions that may certainly result in his dismissal, especially if parents find it and complain (and the termination is supported by state code).