When Ma Bell didn't let people use third-party phones with their service, the government made them change their ways. Why is the government letting Apple get away with the equivalent?
Ma Bell had a monopoly. This is one messaging app on one brand of phone. Nothing stops you from using another phone, or multiple or other messaging apps. And messaging apps aren’t restricted by a “default” in quite the same way a web browser is. I have friends on WhatsApp and Signal and Instagram and I message them all without any appreciable difference in effort.
Interestingly enough, one of the outcomes of that case was IE for the Mac.