So they shouldn't be allowed to pass the savings on to the customers who actually use the service in question and thus benefit from the colocated server? And this is supposed to be more "fair"? And somehow the ISP's rate model for their customers is yours to dictate?

Consider if ISPs actually charged customers by data transfer, the way many large datacenters do. What actually costs the ISP money is data transferred outside the ISP's network, going over peering connections. Many datacenters don't charge for bandwidth within their own network; for instance, bandwidth between AWS nodes in the same area is free. So why can't an ISP use the same model, and only charge for bandwidth going over their peering connections but not for bandwidth that stays inside their own network (such as to the YouTube or Wikipedia mirror they're hosting)?

For the record, I do believe in network neutrality in the sense that ISPs should not be artificially limiting bandwidth to particular Internet services (though I'd prefer to see that solved more naturally through ISP competition, but there's a severe lack of local ISP competition available). I don't, however, see anything wrong with setting up additional peering/colocation/etc arrangements that are designed to be mutually beneficial to the ISP, their customers, and the services whose data they're carrying.

Zero rating is not about passing on cost savings, it's about market power and discrimination.

Also, datacenters do not usually charge by data transfer. Just because Amazon does it (and overcharges by two orders of magnitude) does not mean others do it.