Users with data caps are an obvious reason, for example, see this frustrated user report here: https://support.google.com/chrome/thread/415181794/chrome-al... .

To your point about `fetch` in the sibling comment; yes, I would also be annoyed by a website which downloaded 4GB in the background without asking me, too.

I don't think this is some moral outrage, to be clear; it's well within Google's rights, it doesn't seem "sketchy" at all, and it is kind of a cool feature, but it feels like they could have done a lot better by just making it opt-in and a little less fingerprintable.