Not that it makes it okay for Apple to allow this, but it goes without saying that any app that does this, paid or unpaid, is getting immediately uninstalled.
I've uninstalled lots of apps when they've sent coupons via notifications. Notifications are to notify me about things I need to act on. They are not a marketing channel. So, yes.
That depends. A business that uses notifications for marketing isn't one I want to do business with. If there's a decent competitor I'll switch. If there isn't then I'll decide to begrudgingly put up with it (and probably complain a bit on social media) or to do without it.
So far I've never put up with it and always either switched to a competitor or abandoned the whole thing.