50% of it is tribal signalling (I'm going to define that as meaning that people aren't ACTUALLY invested in the object level claim) ...but 50% (or more, I fear) is ppl who honestly think "my 2018 ethics are self obvious, and anyone who chose differently in 1950 or 1850 is evil"
this also implicates the NPC meme - people who are incapable of going up a meta level and understanding the degree to which they're programmed by their culture