discrimination and what to do

I am entitled to my opinions as you are entitled to yours but your prior comment, while possibly not intentional, came off a little insensitive which is why I interjected.

By bidding them "farewells in the language they've used, without accent" causing them to "turn white and walk away dumbfounded", you have in effect, retaliated, albeit in a non-aggressive (maybe passive-aggressive) fashion. By doing this, you have demonstrated what I suggested earlier. You have educated the culprits in question by

I'm sure that you are already aware but it's a common thought in Japan amongst the Japanese, that foreigners cannot speak Japanese. As a result, some Japanese people will make underhanded comments thinking that foreigners cannot understand them.

You're right. You cannot change the minds of people in a day. And you're completely entitled to your decision not to want. But considering the amount of misconceptions that some Japanese people have about foreigners, I want to promote a better image and I don't think there's anything wrong with that.