"People" don't do it; HN does it. Indented lines in a post are always monospaced, on the assumption, I suppose, that they're likely to be code examples. There's no way to turn this off AFAIK.
I know the formatting system. My question is why the users are formatting their text (e.g. with 4 places leading each line) *suc that HN renders it as monospace, which then forces a sideways scroll on mobile and small screens. That seems to make it harder to read for some with no corresponding upside.
And no, HN does not use Markdown.