It's Apple's product and Apple's store. The idea you can force a company to sell anything doesn't sound very cool to me. If you don't like that, you are free to choose a competing product or build one yourself.
The problem is not what Apple does or does not want to sell in their store. Apple is--and should be--free to choose what content to allow or not allow in their store. But, Apple should not get to choose what content to allow or not allow my device. Yes, Apple created this piece of hardware, but once they've sold it to me, I own it, not them. Therefore, I get to choose what content to store or not store on this device, not Apple.
The fundamental problem is that Apple has set up its store as the one and only store for these devices. Other stores are not allowed. By explicitly preventing me from accessing other sources for this content, Apple's decisions over the content in their store directly impacts the content available for me to load onto my device. If Apple wants to limit the content in their store, they should also allow other stores to exist. Conversely, if Apple wants to be the only store, they should allow all (legal) content into the store.
let me know when you open a store someplace so I can demand you sell my porn...otherwise I'll complain you are censoring my porn.
Let's use your example. Let's say I run a store that sells magazines and you want to sell pornographic magazines. One option is to sell your magazines in my store. Let's then say that I refuse to sell your magazines in my store. Now, the question is: Do you have another option? In the real world, you would have other options. You could sell your magazines in someone else's store. You could set up your own store and sell your magazines from there. You could stand out on a street corner and sell your magazines.
But, suppose my store is the only place that is legally allowed to sell magazines of any kind. In that case, if I refuse to sell your magazine, you have no other options. There are no other stores. You can't set up your own store. You can't sell these magazines on your own. You are out of luck. The people who want your magazines are out of luck. Because my store is the only legal source of magazines, my refusal to sell yours is tantamount to censorship.
That is the situation with Apple and its store. Because they are the only legal source for these apps, every refusal is, in effect, an act of censorship. It is censorship because Apple's denial does not just prevent an app from being sold in Apple's store, it prevents that app from being sold at all.