selling it cheaper
Amazon may not be doing the same thing at all.
The Amazon T&Cs may restrict a seller from offering a lower wholesale price elsewhere (not sure what the actual language says) but that is very different than Apple's attempt to control the retail price to consumers. Apple's policy is to prevent price competition across all channels. The Amazon policy may not.
The second point is that if Amazon is cheap enough, no one else may have much an incentive to sell it cheaper anyway.
Generally, it is fair for a wholesaler to set their wholesale price. But, even they can get into trouble when they try to control the retail price. And in almost all markets the wholesaler can not control or limit the retail price of their products. Apple clearly demanded the right to control the retail price through all channels including direct sales. Or, wholesales forgo the Apple marketplace.
It is combination of this attempt plus the 30% cut that forces high prices to all consumers (Apple and others). That is what is harmful about the policy. And very likely illegal because it does set a high fixed price for products for all consumers.
Many retail sellers might be willing to accept a lessor cut than 30%. But, Apple illegally precludes it via their favored nation policy and high markup.
What does Apple do for it's 30% cut. Practically nothing. Brink and motar stores actually buy inventory, stock it, advertise it, give it shelf space, promote it and handle the merchandise. Apple does none of that. Only the electronic delivery. Yet they demand a high market AND preclude other perhaps more efficient channels from offering price competition.
If you can not buy it cheaper somewhere else, you are paying too high a price. And that harms all consumers simply because Apple wants a 30% cut and wants to preclude price competition. So you get high retail prices fixed by agreement between Apple and some distributors. What's not to like, unless you are a consumer being screwed.
Jerkheads run Apple. And they have been caught out.
Of course, any wholesaler wants a high price too. And Apple wants a high 30% cut. (Did not Google suggest 10% was more appropriate?). And Google is not trying to control the retail price through other channels. And it sounds like the Amazon deal only affects the wholesale price not the retail price. Apple wants to illegally control both the wholesale and retail price across the industry. That way it can collect 30% for doing nothing.