Let me prephase this that I think that the ban seems absurd, both from a constitency angle and even according to the rules themeselves.
I'm going to however agree with gemm in that I find "kys" to be something that is extremely mean to say to someone, and that to me is way above "normal" banter/trashtalk. After seeing the numerous responses on this thread where people described this word as "shit-talk vernacular " and generally that worse things get said all the time, I understand that this seems to be a cultural thing, as playing in EU it is something I encountered very rarely.
I have no doubts that some people managed to dissociate the word from its meaning. But the raucous defense of it (and other things who are also imo pretty bad) as a part of "trash-talking", which is apparently a normal part of video gaming, irks me. I'm sure that for 99% of people who play comp; the best memories made by playing stem from friendships, bonds made when playing. And that just because this form of trashtalk is socially accepted right now does not mean that it should be, and that we'd all gain more, as a group from trying to be a bit nicer.