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Beeper vs Apple battle intensifies: Lawmakers demand DOJ investigation - Android Authority
(www.androidauthority.com)
This is a most excellent place for technology news and articles.
Yes, this part is legal and fine.
This is not allowed because Apple doesn't want to allow it. They own the infrastructure serving the API, they get to determine who is authorized to use it. They can block whoever they want. And technically speaking, using it in an unauthorized manner could even rise to the level of a criminal violation of the CFAA.
Partially correct. It is not impossible to do as I suggested, because I never suggested that they should have interoperability with iMessage.