Because many people here are mindlessly reactionary to the mainstream and the current mainstream is pro-ukraine. It's a little thin now that the war is fucked, but early on you'd definitely see posters giddy at the idea that redditors would go to ukraine and be killed by russian military might.

In this thread, someone has said that the nuclear annihilation of the UK would not be unreasonable. If you keep talking to people they'll tell you. Just ask.