How would you stop them?

The license is Apache 2.

That's my question -- why license as Apache 2

What license would allow complete freedom for everyone else, but constrain Amazon and Cloudflare?

They could just create a custom license based of Apache 2.0 that allows sharing but constraints some specific behavior. It won't be formally Open Source, but will have enough open source spirit that academics or normal people will be happy to use it.

The LLaMa license is a good start.