Note that this was on Dropbox, not on any of the mail providers. Unless Google (et al) is going out of it's way to send emails to its users in the list, which would surprise me.
I think the point was that many people reuse their passwords, so if Google et al were proactive about it, they would force reset passwords for the emails in this leak. But I agree, that may be over-protective/paranoid in some sense...