It's uncommon for businesses to see a client flirting with another vendor, and in response cease all communications. Usually it's just the opposite - wow the client with attention, and woo them back. Apple certainly has the resources to shed clients at will, but it's still childish behaviour. Why make it harder for a client with a good history to develop on your platform?
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.
