We live in a town with just 2 builders.
You can only buy a house from one of them (Builder G and Builder A).
If you buy a house from Builder A, you won't be able to install any appliances or make any changes unless approved by A.
To get into A's approved list of products/applianced, you need to pay them a 30% commission...
This would never be accepted in the real world, but we go for it on the App stores.
Not the perfect analogy, but explains what happens with Apple and Google.