Fortunately, most sites don't do this. I don't understand why it's done in the first place, though.

It's mentally agitating when I go to scroll on a webpage and it doesn't do what I expect.

And what content authors should be most concerned about: it distracts viewers from the site's content, because I'm thinking "what just happened to my mouse". And that reaction is the antithesis of what content creators should want: they should want to engage me, and explore the site more deeply.