One example that always springs to mind, was Australian friends online in the early 2000's who told me their ISP had linux mirrors - which was of great value to them, because they were charged less (if at all) for data within their ISP's networks. They were also much faster, since it meant the data wasn't trans-oceanic. Later Steam came up as another example of the same idea.
They thought this was fantastic. It solved a lot of problems (and expense, and speed) for them. But it's not particularly 'neutral' either, is it? Given the choice between a distro that's on my ISP's mirror, and a distro where I have to slug data from the other side of the planet, I'm going to make a choice based on my ISP's preferences.
Putting a wikipedia cache/mirror within the ISP's network would, in this example, render pretty much the same results as "wikipedia zero". Same benefits, same concerns.
I'm not really arguing this one way or the other. Just that it does muddy the water when we have examples that benefit the enduser, to counter the scarier examples we see elsewhere.
There was an attempt by Facebook to introduce zero rating in India through a big telco, which failed due to the protests... and guess what? The telco launched high speed internet services free for an entire year(google Reliance Jio) and even after that its almost free(1$/month/30gb), without any limits whatsoever.. had there been zero rating, they would have simply not released the lower priced service..
If your question is why shouldn't tellcos be allowed to give wikipedia away for free, then that is a more complex question that requires a more complex answer.
But it is a question that is entirely unrelated to the present net neutrality debate as it is happening in the US. The present debate is about telco services for which people pay a lot of money for. It is not about connecting the previously unconnected poor but about limiting the connections for people that are already connected and expect to stay connected.
Perhaps there should be an exception made to net neutrality for services for the poor that are free of charge. The same way food shelters do not have to comply with the same laws as restaurants. Fine by me, but this is beside the point and not germane to the main net neutrality discussion.
Links are the essence of the Internet. Internet broken up in unclickable silos will have huge effects.