You do realize that it forbids your phone (or the computer you tether) to be a server, right? That means you can't receive calls over IP without the need for a third party, for instance.
So I'd say they do care about how you use their service. They wouldn't want to give you the means of destroying their busyness model, now do they?
I am perfectly well able to receive SIP calls over that private IP. Sure. You do need STUN, but honestly, I have a feeling that it's ever so much easier to do correct roaming between cell towers with those internal IPs which is likely the reason for this configuration.
I don't know how roaming is supposed to work, but I'd be very surprised if your feeling was grounded. Here's why:
(1) You keep the same phone number, wherever you are.
(2) Calls don' end even when you move.
Which means they can keep track of state, and link that state to your phone. Id' be very surprised if the same kind of mechanism couldn't be use on IP technology. My bet is, they do: have you ever noticed having to constantly reconnect when you drive in a well covered area? If not, that most probably mean that they keep track of the NAT table associated to your phone, and of course your private IP. In that case, I can't fathom why they couldn't do the same with a pubic IP.
You do realize that it forbids your phone (or the computer you tether) to be a server, right? That means you can't receive calls over IP without the need for a third party, for instance.
So I'd say they do care about how you use their service. They wouldn't want to give you the means of destroying their busyness model, now do they?