What are they stealing?
By allowing Android <--> Apple communication, Apple has potential "loses" as they loose their lock in ability of forcing people buying a new iPhone for Christmas.
iPhone $800, Android $200. If an Android can do what iPhone can do on the messaging front there's no purpose in buying an iPhone.
And that's a problem...why?
Comparable devices cost pretty much the same whether iPhone or Android.
While most people may not care it’s also worth noting that the alternative OS (Android) is operated by one of the largest ad-tech companies in the world and allows for deep oem integrations of 3rd party applications (that 99.9% of its users aren’t even aware of as being installed.)