I’ve just checked my test site, and I believe the default for chat email notifications is ‘only when away’ (with the only alternative being ‘never’).
I think there is a case to allow for a default to be added to the User Preference admin settings, though probably for the inverse of what you’re suggesting. 
This would work like the other defaults, in that it would only change the preference for users who had yet to select one manually.
I’ve slipped this over to Contribute > UX for consideration. 