Historically, professions are regulated and licensed for the very reason that the professional is obligated by rules of ethical conduct which do not permit him or her to perform below a certain legal standard of care. For example, a doctor cannot turn someone away from the emergency room and is obligated to preserve life at all costs, and thus cannot generally say "He's not worth saving," even if the person he's asked to care for is a terrible person. A lawyer is obligated to "zealously" defend his client, even if that client is a serial killer. An accountant has strict responsibilities to properly keep books of record and manage money in his care.

When someone is asked to "act professionally", it means that a standard of care is expected beyond what would be required simply for responsibly performing a job. The borders may be blurred a bit these days, but that is the intent.

Thus, if teachers were "professionals" then they would continue to teach even if the union contract was in dispute due to their licensed standard of care. I suppose that doesn't exist right now. Maybe it should.

Regardless, that is the point. I have to be careful when taking on clients because if, halfway through a matter, a client stops paying, I have to follow strict rules about continuing to represent that client until a judge permits me to cease representation (or new counsel is obtained). That's not something that most people have to worry about.