Actually, FCC has chimed in on this 'zero rating' https://www.theregister.co.uk/2017/01/12/new_fcc_guidelines_...
FCC liked what T-Mobile was doing with Binge on because it didn't favour particular services or alter consumer choice significantly. It was fair and open or any website to participate in
> When it comes to T‑Mobile US Binge On services, FCC staff decided that the service "did not compel edge providers or consumers to participate in Binge On, and did not charge them anything if they opt to do so" – and therefore was legit.
> However, AT&T's Sponsored Data program "presents competitive problems" because it "imposes hefty per-gigabyte charges on unaffiliated third parties for use of Sponsored Data" – charges that "far exceed the costs AT&T incurs in providing the sponsored data service."