It is quite simple realy. If I pay x€ for y GB/Month, the ISP should not dictate what over the top services I spend that budget on, nor favor its own services (or those of 'partners') by selective exclusion of that traffic from limits that other are held to, or by hindering traffic from the non-partnered services in any way.
Now for the 'complicated' part. In practice it is not difficult to create a situation where everything is degraded. You just under-provision on a choke point. If you then sell alternative routes, or allow edge caches beyond the choke point, you technically didn't 'hinder' any traffic, you just provided such a lousy service to begin with that any not otherwise enabled service provider doesn't stand a chance of offering a decent experience on your network.
But just pointing to a technical situation where it is hard to write a general 'rule' that in any arbitrary case can unequivocally objectively and automatically say whether a certain criterion was fulfilled, and then saying that because it is hard or even impossible to specify the whole regulation should be scrapped, that is disingenuous.
The spirit of the regulation can be perfectly fine, even though case-by-case judgement may be required to determine compliance.
Those that will say 'but you can vote with your wallet', I can see where you come from, but the reasoning is flawed. In practice telecoms is not an open market, and many households have near 0 meaningful choice. Strong economic network effects are inherent, and lasting competition has only been present under very strict regulation.