Marketing
Remember, Apple is a for-profit company. If people exclude and tease Android users, Apple stands to benefit. With that in mind, it makes sense that iMessage would be designed to exclude Android users.
Now it’s going to get a little philosophical. Do marketers have an obligation to society? Is it their responsibility to consider the negative effects of their marketing? Anyone could answer that with their own opinion, but if you ask me, no. They’re trying to make money, they’re going to do what they can legally do. So if it’s not the marketer’s responsibility to take society into account, it becomes society’s responsibility to take marketing into account. It’s more than billboards and sponsored posts on Instagram. Marketing infects the way we perceive the world and interact with each other.