Alternative browsers--built using Apple's browser--exist on the App Store; the issue here is that users are paying for something via a mechanism that isn't in-app purchases: Apple wants all payments in the world to flow through them, and so the idea of a decentralized payment future is incompatible with the App Store.
How is this any different?
This is yet another reason we must fight these proprietary app walled gardens. It's a clear regression from the open web.
Excerpt (in the context of Ethereum, a smart contract platform):
> A DApp has its backend code running on a decentralized peer-to-peer network. Contrast this with an app where the backend code is running on centralized servers.
> A DApp can have frontend code and user interfaces written in any language (just like an app) that can make calls to its backend. Furthermore, its frontend can be hosted on decentralized storage such as Swarm or IPFS.
> If an app=frontend+server, since Ethereum contracts are code that runs on the global Ethereum decentralized peer-to-peer network, then: DApp = frontend + contracts
Instead of talking to an API connected to a database, it's connected to an ABI talking to the blockchain. Someone could directly interface with that, or use my client, or build their own etc, it's open on the blockchain. Read operations are free unless specified, write operations generally have an implicit cost of the smart contracts currency at least with what I've used, which was Ethereum.
Exceptions list:
1) Content served over HTTP(S) such as JavaScript.
2) WeChat.
3) ...
Most dApps (at least the ones you access through a dApp browser) have UIs built using regular web technology, and served over http.
As for why Google/Apple are doing this, I can only imagine that it threatens their walled garden by giving web users more access to payments that they'd rather monetize.