Is this specifically unauthorized, though? The user is permitted to use Apple's services, and Apple has, as far as I know, not announced that third party apps may not use their services.
With the trouble Apple goes through to ensure you are accessing APNS from an Apple device including obfuscating the signing algorithm and requiring unique hardware identifiers I think it’s safe to assume they don’t want 3rd parties accessing their services.
It would be nice if third party clients were allowed to connect, but it's totally understandable if they don't want to allow it. Servers cost money, and misbehaving client apps that you have no control over sound like a pain in the ass.
i’ve run multiple 3rd-party signal clients, even alongside the official apps, and never seen any problems or warnings.
Moxie (Signal's founder) has thrown fits in the past over the existence of third-party clients using their servers: https://github.com/libresignal/libresignal/issues/37#issueco...
That was pretty damn polite for a heated mailing list discussion.