Gosh I hadn't seen this. I really feel for you and have had a few incidents myself, , as has my boyfriend (latest one for him was just last night).
I dunno what it is, but it seems as a mtf if someone slips up, and I correct them, they're generally embarrassed. With my ftm boyfriend, however, it's completely different...they act like this ignorant monster you encountered. I just don't know why that is.
I am glad you and your mum are pursuing a course of action to invoke change. I had a couple of times where I should have done something similar, one at work especially, but I was too much a coward. Good on ya!