I don't see why anyone would expect this closed-beta product to not get blacklisted by Apple as soon as it gains any mainstream traction.
The disadvantage of not offering an API for iMessage is that there is no easy way to block clients. Apps like Bluebubbles have to work by being installed on an actual Mac and scraping messages from iMessage and Beeper uses a similar architecture[0]. Presumably they have a data centre full of actual macs (or VMs). [0] https://github.com/mautrix/imessage
Sure there is. Remove the client from the app store. They could even remotely uninstall the clients if they wanted.
>Presumably they have a data centre full of actual macs (or VMs).
Apple could ban every single one of those machines from imessage.