It's a net win for the ISP due to cost savings from not having to haul that traffic across their network/to the Internet.

You're right that powering racks isn't free either though: if the cache operator doesn't utilize the rack well and isn't offloading enough traffic to it, the ISP will give them the boot (as in, "we wheeled your rack out to our docking bay, come pick up your piece of junk if you want it back"). It's uncommon since _most_ of the orgs running these racks are competent, but does happen from time to time.

Also, even though it's a net win for the ISPs, there are still cases where the operator ends up paying them a fee. This has less to do with the economics of edge caching within an ISP network and more to do with the bargaining power certain ISPs have. The 2014 Netflix/Comcast peering agreement is a good example of how those things sometimes pan out.

MSN paid. We also carried all traffic for Microsoft properties; if you’re an ISP, whatever you were paying for Microsoft data went to zero and your other links had more capacity.