you'd be billed for 2 connections, currently
1 for the 10mbps service, and 1 for the max service (which would also come with it's own highspeed as well)
there has been talk of allowing case by case exemptions, but I personally don't really see it happening.
ie, if your modem syncs up at 50+ mbps and you only need 25-35 for your TV streams, why not allow an upgrade to 10mbps highspeed on the same modem?
I don't know or claim to know exactly the reason why it's not, but I suspect its just to offer a uniform product, and message to customers.