Imagine that you paid $1000 for a 1 year CostCo (iOS) membership which gives you access to CostCo but only on the condition that you live in CostCo Village, where you can only buy Certified Approved™ products from CostCo (which have a 30% markup) and if you choose to leave, you'll lose everything you bought while you were there. Home, furniture, electronics, everything. CostCo Village is a nice place to live and the products sure are nice but imagine 50% of the population living like this. From a consumer perspective it's shitty because you have no choice and from a vendor perspective it's shitty because you either play ball or lose out on half the population.
Sure there are Sam's Club (Android) Villages where you can still buy from other chains or independent grocers but it's just stupid that you have to make this choice.
Another thought...we live in a capitalist society (assuming you live in the US). With the customer base that Costco has, you can’t realistically expect them to give away the most valuable thing they have to vendors, can you? Sure if you run a single convenience store you can’t charge for shelf space, but if you run 1000 of them you probably can. So if you’re Microsoft for example, you need to beg developers to build apps for your mobile OS because the value proposition i.e. paying users just aren’t there. Flip that story on its head and you have Apple.
Also, I don’t quite understand what you mean in your last point....
Apple doesn't pay them. Anyone who wants to sell media on iOS has to compete with iTunes while subject to a 30% price handicap. If that's not anticompetitive I don't know what is.