No sarcasm, and that question wasn't rhetorical either: How is a scrollbar that different from other input elements within webpages that can already be styled?
And what do you mean by using non-default colors? User stylesheets would also include scrollbar colors anyway; do you mean just setting a background but no text color (assuming a default of black, which might not be black for the user)? Don't see how this could apply to this CSS property, since it takes two colors, you either set both or none.
It seems logical to me that if you can read the text on the web page, that is, if the web dev hasn't made the common mistake that renders the text unreadable, they will set the scrollbar colors to something of similar "quality". If you can't read the text because they use CSS badly, then the scrollbar color is the least of your worries. QED?
Generally speaking, there are a lot of "common webdev mistakes", like using the giant images and scaling them down, having text and background color very close together... but we don't take that away just because some abuse it, so resisting more flexibility on those grounds doesn't strike me as so "obvious" as people seem to agree it is. If you can allow websites to hide the mouse cursor, I don't think the web will break because of this CSS property. If you don't want to see it on any page, override it in your user stylesheet, done.