They were testing it last year, you could clearly see these headers on a large percentage of traffic coming from their gateways.
I'm not expressing an opinion one way or another but they clearly felt the UID is not directly identifiable and thus does not become a privacy issue until they share the mapping of the UID to customer data.
My guess is in their minds if you opt-out they just do not provide your UID to 3rd parties for targeting.
In the ever increasing dream of cross device marketing (think your iPad, iPhone and Laptop) many companies are trying to figure out ways to connect these devices to a single individual or family.
IIRC Verizon quietly started rolling out service wide TOS changes to allow this sort of thing a couple years back. That said I'm not sure if their TOS makes it clear how this is implemented and what potential side effects might be caused by the way they've implemented them.