I think the returned civilians find the term that the Autobots are using offensive because the Autobots are soliders using it as a derogatory slur against civilians. It's the tone and context not the actual word, just like a lot of otherwise harmless english words are used as words of hate towards people from different backgrounds, religions, orientations, etc.
And if Prowl intentionally wanted a word that was insulting he's smart enough to find a couple of Cybertronian words to form an appopriate accronym.