Genuinely, why do they do this?

Are they actually Russian? In high school I knew a few weird dudes who tried to learn Russian and essentially cosplayed as Russian like weaboos for eastern europe. 

first of all some browsers have auto-translate function which is actually confusingly good, to the point where you can accidentally think the original post is in Russian, for me it usually happens when I open reddit post through Google and forget about it applying automatically (even more weird, it doesn't happen consistently, sometimes it opens translated, sometimes it doesn't so I have to check every time).

second, sometimes you just want to say something people will understand only with cultural background similar to yours, for example if you see something that can be a reference understood only in that language (that's a rare occasion though).

third, some people can just simply be jerks.

I often see comments written in spanish, french, german etc on English-speaking subreddits and posts written in English, so it's not like only russians do it

А чтобы вы не расслаблялись

do you translate your comment to russian when you reply to the russian comments?

да не, бред какой-то

А потому что так надо, нафига америкосам знать наши "секреты"

(ok, speaking for real, probably because of lack of English knowledge)

Потому что понабирают долбаёбов

постараюсь так больше делать 👍

Вы не поверите, но играя в ммо на русском сервере, обязательно найдется англоговорящий долбонавт который будет писать на английском и какие-то претензии свои пихать.

One the one hand, yeah plz speak english in english online communities. On the other hand americans should probably keep quiet about this one lol, I'm absolutely certain there'd be a lot of them doing the exact same if english wasnt the default online language.