Earlier quoted context omitted.
This aspect is ignored, because it's clear that Apple blocks third-party clients to maintain its dominant position in the US (social unacceptability of green bubbles among teens). If cost was the problem, they could offer a subscription.
It's pretty clear why they don't want an android iMessage app. In this case, what beeper enables (if successful) potentially is to use Apple's infra for future communication between android to android phones, or android to iMessage groups, while on Apple's infra and dime. Beeper will likely collect a fee for it as well. Thats not a position Apple would want to be in.
The only reason is to bully people who fear social exclusion into buying an iPhone.
(I am an iPhone/Mac user, so I am not trying to bash Apple from the other side of the 'divide').