From what I remember reading when Wehe came out, ISPs in Spain and Portugal (and Brazil?) were already providing data capped plans with options to uncap particular services for a small fee. So you'd get an (example) 100GB plan and pay $1-2/mo for each service you want excluded from the cap (Netflix, or whatever).
IANAL but I'm sure this was/is legal in those jurisdictions, which indicates a total disregard for net neutrality there.
So, I can have 100Gb of data for X or 100Gb of data plus all-I-can-eat from 'PopularService' for X+Y. If I choose to pay extra, that's my choice. Seems reasonable, after all it's my own bandwidth that I'm buying with my own money.
It's dodgy if the ISP serves it at lower speed for either capped or uncapped. An ISP might throttle uncapped to force the service to use lower quality and less bandwidth, or they might throttle capped to encourage folk to pay for uncapped.
It seems neutral at first glance, but the side effect is a barrier to market for competitors not on the ISP offers.
As for legalities until it's tested in court it's hard to know if any law stands up to scrutiny especially in an area as grey as this. My gut feel is that ISPs would get away with it unless they are abusing a monopoly, but hey IANAL either.
In the US most mobile carries will hijack your traffic, either slow it down(they call it whatever saver), or replace the data with their cache.
I tried a T-Mobile MVNO or T-Mobile(?) phone a few years ago, noticed regardless whatever resolution I set inside the Youtube app, it is super blur on Cellular but it works fine on Wi-Fi. The I found that the carrier setup its own cache of low resolution youtube videos, it will hijack all your requests to youtube and replacing the content to its own cache (360p videos).
That's how they give you "unlimited" data as you will never able to use it.
Is this some sort of a US ISP problem that I am too Eastern European to understand?