I suppose it's like any other paid app you would buy from the Google PlayStore, once you've bought it, you own it and don't have to pay for it to install on other devices, or reinstall on the same device after it's been deleted.
PROVIDED IT IS THE SAME APP.
If a new version is introduced into the Play Store (or Amazon App Store) as a completely different app, then it wouldn't matter if you bought the previous one.
It's the same with the Apple AppStore.
I'm an Apple user, but I have friends and relatives that use Android devices and I must admit the Android MB3 Client app is a thing of beauty. The iOS version has a lot of catching up to do.
MB Team, Thanks for you generosity with this! Love this community! Will be donating again.