The cost savings from CDN colocation and peering to the ISP as so miniscule that, even if they were passed on in full, would barely register on your bill. We are talking about fractions of a cent per customer.

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.