I guess it's an instance of a more general principle: sending an email doesn't guarantee it gets to the user's inbox, never mind that it gets read.

Even if you are OK with the idea that a user can be presented updated TOS with no option to disagree (I don't, but put that aside for a moment), it should still require a mechanism that actually guarantees (or at least verifies) that the user has seen that the terms are updated. Email is not that. (An unskippable notice on login to a web service would be.)

If registered mail is sufficient and that only requires proof of delivery/receipt, why would the same thing for email be insufficient?