Exclusivity arrangements are nothing out of the ordinary between businesses. Developers are not regular Apple customers.
What is out of the ordinary is this way of doing it. Simply not responding to emails any longer, without explaining why, is very unprofessional indeed.
If there was some kind of exclusivity agreement in place, it would make perfect sense. "As long as you're publishing exclusively to iOS, we'll give you extra support and guidance" is fine if both parties are aware of it and have agreed to it.
"Nice app you have there. It would be a shame if you stopped getting support on it" is not.
What is out of the ordinary is this way of doing it. Simply not responding to emails any longer, without explaining why, is very unprofessional indeed.