I suspect a lot of is that people formed that opinion of Microsoft in their formative years. This generation we commonly see on HN has this childhood definition of Microsoft as evil internalized at a pretty deep level. People look for reasons to justify their beliefs more than they look for reasons to challenge them.

There are companies I got a bad impression of many years ago too that I am hesitant to reevaluate. I think it's pretty normal/natural, even though it's kinda unhealthy from an intellectual standpoint.