There could be many causes of what appears to be differential throttling.
For a small ISP there might be a much higher quality backbone link to one video provider than to another. This is probably a coincidence.
Also, popular services such as Netflix result in the build out of more dedicated bandwidth that is shaped to match peak demand.
Broadly, traffic shaping happens from QoS rules, ISP upstream provider choices, backbone infrastructure build-outs, etc. It's not a static system.
I guess my point is that this is too complex a thing for regulators to understand or to regulate in a sensible way. I prefer simple antitrust enforcement of last mile providers, even if it entails the government "confiscating" the transmission lines installed by ISPs and any of several ISPs to use them to serve a customer based on the customer's chosen ISP.