There is a difference between shaping traffic to better fit the capacity available, and constricting traffic since you're incentivized to push alternate traffic from a business partner. The latter is what NN advocates are trying to discourage, and it does arguably mess with the 'level playing field' notion.
I don't think this distinction is as trivial as you think. Especially in a legal sense. Unless you want judges adjudicating in the Potter Stewart "I know it when I see it" sense, which can get scary.
The legal system is built on intent and purpose, so from a legal sense, such distinction is not trivial but its quite normal. What we as a society might want to avoid is ISP that calls a company and say "Would it not be terrible if you lost access to a third of the population in this country. Pay us now, or we will redirect those users to a competitor."
If your traffic shaping rules are different depending on protocol or port number or source or destination IP or AS, then that's discriminatory. Within the space of packet scheduling policies that treat all protocols equally, it is still possible to be somewhat discriminatory and it is still possible to do good QoS. Further and more careful regulations may be necessary, but it is certainly possible to cleanly prohibit a large category of bad behavior without having harmful side effects.
