Shouldn't you not be attempting to modify the appearance of the scrollbars and letting them be the style the OS chooses, if you want them to look "native"? These developers seem to be wanting scrollbar styling so they can make scrollbars look different from their native appearance.
There are also quite a few truly native apps which have reinvented scrollbars instead of using the OS', and they are awkward to use because they don't behave exactly like native ones do.
Behaviour is not the same as styling. A good app's scrollbars will behave natively but will look in line with the rest of the app.
Compare GMail's scrollbars, especially the ones for the XMPP Chat, between Firefox and Chrome.
...or are we not agreeing on what "native" means? I've long thought it referred to the default appearance/behaviour of the OS/browser.
But scrollbar styling is very different to scrolling logic. We're not talking about stealing people's mousewheels here, we're talking about changing scrollbar themes.
This is important for apps which want native scrolling logic inside their app but want their app to look native. The only alternative, in fact, is to reimplement scrolling logic inside the app. Performance suffers, accessibility suffers, everyone loses.