Can the electric company ask for a 30% cut on anything you buy using their electrons? People would just build their own distribution lines to the electric company down the road if they didn't like it.
How about your landlord charging a 30% cut if you want to take your amazon package inside his house? Don't like it, go build your own home.
That actually happens in commercial real estate: https://en.m.wikipedia.org/wiki/Percentage_rent
Would not buying an Apple device put you in a similar position as having no access to electricity? Can you not buy a different and phone that allows you to freely install any software you want too without having to pay 30% to any third party?
And it's the user's phone, which they should be free to use however they see fit. Even if that means installing software from places other than Apple's store.
"Just buy from someone else" works great for buying furniture or socks, not so much for home electricity or smart phones.