They are already in it. Google for verizon supercookie

That train left the station years ago.

Verizon Customer Proprietary Network Information Policy: https://www.verizon.com/about/privacy/customer-proprietary-n...

With wireless, all of the carriers transparently proxy most traffic and can do a variety of things with it.

See: http://www.cs.usc.edu/assets/007/90818.pdf

They could be in the marshmallow manufacturing business for all I care, if it's not their core competency they'll suck at it. (There are a lot of third party ad-tracking/online surveillance companies to be more worried about since that's their entire bread and butter.)