Would you care to expand on these bad precedents that you are referring to?
Presumably the OP is concerned that this could degenerate from "breaking net neutrality is fine as long as it favors sites we like" to "breaking net neutrality is fine".
Note that I don't necessarily share the belief of the OP, but it's certainly a point worth some real consideration.
In the old days when city governments granted these monopolies to cable TV companies, part of the deal was community access channels (Wayne's World!). Even if these benefitted hardly anyone, they provided some moral cover and justification.
If I were an old media guy, I'd see Wikimedia Zero as the germ of an idea -- maybe suggesting a way how to squeeze the genie back in the bottle and wrap the internet in a cable TV model. And, hey, in the US most people's ISP is also their cable company. So ... I don't see it as a definitely bad thing, but I see the seeds of potential bad as well as good.
(In general, a lot of bad stuff flies under the flag of a good cause fighting some other bad thing -- "because terrorism", "because children", "because hyper-inflation", etc. Of course a lot of _good_ stuff also flies under good flags. I'm just saying it's not obvious either way.)
The first thing to ask is what differentiates Wikipedia content from the rest of the 'net so that it would be OK to break net neutrality principles towards the implementation of this project.
So, even if this breakage is somewhat worth the bending of the rules, companies must support the idea. Once they do it, what would be the moral ground to rightfully deny another proposals from content providers that offer them money to do that?
Well, this problem is not something really new. Net neutrality is already broken, though almost everybody agrees on its importance. There must be some kind of consistency if we want it to survive.
What if google or bing made a similar deal? Or any large corporation, for that matter? You set up an internet where the majors can buy their way into a monopoly position, not by convincing ISPs to degrade other services, but by convincing them to charge users less to use your service. This makes it harder to impossible for new companies to enter those markets.
All that being said, I think this (Wikipedia Zero) will be a net-positive thing, and I hope they succeed. I do see that this type of thing could have worrying implications for net neutrality, but ultimately they're not the first site to broker "free access" deals with mobile carriers.
Facebook already does this https://www.facebook.com/notes/facebook/fast-and-free-facebo... and I'm sure many others do as well
I don't think that the world is going to end here, or collapse into some dystopian nightmare, but the question was asked: "what are these bad precedents?" and I attempted to answer.