Same reason PS messenger can block third party clients.
Is it ethical? Not in my opinion. Facetime and iMessage were introduced under the guise of becoming open at some point, however that was because they lost a patent lawsuit iirc[0]
it's worth focusing on the differences between iMessage and Internet Explorer (which was the last monopoly battle that anyone cares about).
The truth of it is: across Apple devices, while iMessage is installed, the alternative message systems are just too successful to argue. The overwhelming majority are using facebook messenger or whatsapp anyway. So there's no evidence that Apple is curbing another industry.
Apple is also not in a monopolistic position except on their own hardware, which is true for everyone (including Blackberry with their messenger).
I am struggling to see how this is comparable. Allowing iPhone users to talk to Android users through iMessage is a benefit to iPhone users. iPhone users get more end-to-end encrypted messages.
Why do people continue to think that you have to have a monopoly to be monopolistic? Also, Apple does have a monopoly. They have a monopoly on all devices that can communicate through iMessage.
If that should be taken up by the DOJ or some other body is up to your interpretation of the law.
