Well, they might have wanted to keep everyone on unlimited phone data but reserve the right to handle MHS/MBBC however they like. So they separated them.
Funny about pro-rating - typically, in a metered environment (like voice minutes or a Tiered Data plan with limits), "pro-rating" is applied to the units used. IOW, say your are authorized 4GB/mo and you only have the service on your account for 2 days and use 2GB on each of those two days (4GB). Well, when pro-rating, you would only be authorized 2 days divided by 4GB/mo. IOW, you would be authorized 0.133GB per day for 0.266GB total for 2 days - so, you would have a 3.73GB overage to pay at the overage rate.
This would have to be completely different with an unlimited plan. I suppose they could pro-rate days only like they used to for the old VZWAccess tethering plans. I dunno... Why didn't they call it a "feature", like before?
I mean, that would mean you could hold the tethering add-on for two days, for $2.00 total, and pull down 40GB, no problem. Just sounds to good to be true.
We'll see soon enough.
-Frank