The fireman in MN had absolutely no right to fly a Confederate flag on what the OP calls "his fire engine". The choice to put that flag on the truck at this time is a very very clear political statement supporting that flag. I don't think he was just supporting the Dukes of Hazzard, but it does not matter what I think.

The truck is not his personal billboard. The truck belongs to the government and is absolutely not "his fire engine" If the fireman had instead chosen to fly the hammer and sickle flag of the old USSR would that have been ok? If he had chosen to fly the rainbow flag of the gay right's movement, would that have been ok? He can do what he wants on his own car, and take the flak he gets for his decision as a private person. But he has no "freedom" to use the government's vehicle for his personal speech. That he even for one moment thought that adding the KKK's favorite flag to fly alongside the US Flag makes me seriously question this person's wisdom.