Beeper isn't "reselling infrastructure" they're building a compatible clone of Apple's infrastructure. And yes, a variety of companies have reverse engineered and copied Amazon S3 for example.
The difference is that Apple's business model is built around preventing you from communicating with iMessage while Amazon is happy to let you copy things from anywhere into or out of your Amazon S3 bucket (for a price.)
This is not correct. The Beeper client directly connects to Apple's servers while bypassing authorization checks through serial number spoofing. They did not build their own infrastructure based on reverse engineering iMessage.
You have to do that to communicate with iMessage. The infrastructure is the Internet, Apple is just operating a mailbox and the only way to send a letter to that mailbox over the public infrastructure is to bypass the auth check.
The difference is that Apple's business model is built around preventing you from communicating with iMessage while Amazon is happy to let you copy things from anywhere into or out of your Amazon S3 bucket (for a price.)