Indeed, most large CDNs will ship you, for free, hardware to install at your ISP PoP in order to reduce both theirs and your bandwidth costs. Google does it, Akamai does it, Netflix does it. Assuming a decent cache hit ratio, this drastically reduces bandwidth on expensive, percentile-billed Tier 1 upstreams that most ISPs still depend on - especially in the US, where truly neutral internet exchange points are rare.

Interestingly, the 1TB cap is exactly what ISPs use to plan for and calculate their percentile-billed upstream commitments, as a 1TB cap corresponds to a sustained 3Mbps usage.

This is what the whole net neutrality debate was about. If ISPs cap data for small video sites, but not Netflix, that results in a massive advantage to Netflix (and, presumably, other large incumbents).

For what it's worth, T-Mobile did this with Binge-On: https://www.t-mobile.com/offers/binge-on-streaming-video

What T-mobile did for Binge On is allow any legal video service to sign on and be zero rated. They didn’t advertise it, but they even allowed some porn sites.

There was a poster on HN who said that his little small non profit was able to fill out a form and be zero rated.

ISP charges seem to never have any relation to ISP costs

You just invented "net-not-neutrality" ;)

Sort of, I suppose. It's usually more about peering and less about caches.

Charging based on traffic that leaves the ISP doesn't seem anti nuetrality to me. Charging based on specific peering arrangements does.