Well, presumably if carriers provide reduced- or no-cost access to Wikipedia that's a direct violation of net neutrality, aka charging the same amount of money for data moving through the pipes no matter what it is or who it comes from.
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.
I do think there is some risk (though I'm not sure how much delta it adds to the risk either way) that it plays into normalizing the proposals for tiering, a specific kind of non-neutral net, where ISPs would give you access to different levels of "internet content" based on your subscription level, like cable TV packages for the internet. Example: a few sites are available in the Free tier, a basic whitelisted set of sites (news, webmail, popular blogs, etc.) are in the Lite tier, everything but high-bandwidth video and torrents comes with the Standard tier, and the full internet is unlocked only by the Premium tier. The free tier would be made up of "content partners" who are a mixture of nonprofits like Wikipedia, and for-profits that pay for their inclusion in the free tier (CNN, maybe).
Which sounds like how cable TV works (at least in the US).
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.
I can see some possible issues, too. If you want to take a hard line approach, this breaches net neutrality just as much as slowing or charging access to other sites. Sure, Wikipedia is already the defacto source of information for a lot of people, but this would cement a monopoly as such in those areas. This all sounds a bit silly when talking about Wikipedia, but it has implications for other services.
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.
Oh, I'm well aware. In Australia, I remember seeing a number of the telcos advertising "mobile data with free access to facebook and twitter". One of the catchup TV services also has "unmetered access" deals with a number of ISPs.
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.
If you get access to sites you care about for free, or heavily subsidized. Would you pay for access to full internet? This might give a lot of informational control to governments/internet providers. If let's say Google, and Facebook are accessible, would those companies fight for some libertarian website that was blocked?
