I think you misunderstand, OP is purposefully adding "site:reddit.com" to their search query, to only get results from Reddit. Google isn't adding it for them.

I think you misunderstand, OP is purposefully adding "site:reddit.com" to their search query, to only get results from Reddit. Google isn't adding it for them. I do this sometimes too, because it's one of the ways to get actual opinions on something, and not just worthless blog spam.