Apple develops iMessage, and gives it away for free on iOS, as a way to make its own platforms more attractive.

Let's say this hypothetical "API neutrality" proposal were to go through -- note the actual letter from Blackberry's CEO never mentioned the phrase "API"[1] -- then Apple might simply stop developing iMessage. Unless your hypothetical law forces them to make the API available and keep it available indefinitely to all comers, effectively subsidizing their competitors out of their own pocket.

[1] http://blogs.blackberry.com/2015/01/blackberry-net-neutralit...