It's an obvious slippery slope. Once the network is established, they will likely start offering something like "Amazon Mesh" where Prime subscribers enjoy "additional free WiFi coverage all over the world", because it's just a no-brainer. They might cap speeds as a fraction of what the device enjoys (e.g. 20% max), as to not "disturb" the owner, but that's it.

This has already been done by ISPs in several countries, Amazon is not breaking any ground here.