One, yes crude language is protected speech.
On the other hand people who deal with lots of grief hourly deserve some kind of protection from daily verbal abuse.
On the other hand, some of the abuse is likely attributable to "arrogance" on the public servant. So perhaps these people should receive training and become fireable, or at least reassigned if if becomes chronic. But being public employees and in a labor union, that's not going to happen.
Still, I understand its a tall order to ask people to put up with awful abuse day after day. Imagine you are a woman and the majority of people you process through are unenthusiastic males. You are bound to get fed up.