Their PR lets them have it both ways.
It's the most profitable course of action, that's all there is to it.
But to address your question: if apple allowed other stores, they could be significantly more picky about what's allowed on their store. This would consequently increase the security for people only using the official store.
This is obviously a no-go however, as that would be less profitable then the current situation.
also you do understand businesses need to make profit, and keeping their customers happy is definitely the right way to do it in my book. so given your logic here, how do you explain all of the other privacy features apple gives away for free?