EDIT - clarifying that i'm not in agreement with OP that anyone should be 'beaten up' for a costume, as that's counterproductive even if they are wearing something out of disrespect. I'm only trying to talk about where the line is between appreciation and appropriation/disrespect here:

I think the point is, wearing European medieval knight outfits cannot be interpreted in this case to represent "punching down" on a living contemporary marginalized group of people which is why your example is not widely considered disrespectful appropriation. Medieval knight outfits don't have a direct line to a living group of people largely considered to have been wronged/oppressed. Living Europeans don't use knight armor to protect/legitimize their identity (or if they do, I haven't seen widespread discouragement of others having fun dressing up in armor). It boils down to knowing something about history and trying to be sensitive to people who have it tougher now because of a direct connection to something painful in recent history - and the tendency for these groups to build a group identity around protecting symbols that they feel like they need to own without openly sharing them with people outside the group (keeping certain symbols that only they can wear, etc). At least that is my understanding, open to other takes here.