Professional and courteous behavior in public places is a sign of intelligence and courtesy to your fellow travelers.
Should one decide to discuss controversial topics, then they have to deal with the possibility of someone overhearing and giving his/her opinions to the conversation at hand.

Words are only words. Political correctness for the sake of such may be your thing, but it is not worth fighting over. If because you intervened the discussion happened to escalate to harsh language between you, chances of physical violence can arise. Then you have a whole new ugly world of troubles to deal with, where you would potentially be viewed by the courts as the perpetrator. Bad news all round. Unless you enjoy jail cells and court rooms.

I tend to focus on other positive things in my life. I tend to mind my own business. My earbuds are my best friend in such circumstances.