[kjksf]

"is it justifiable" is an irrelevant question.

Apple charges 30% because they can. They can because software companies have demonstrated they are willing to pay that 30%.

Apple holds all the cards so they have pricing power.

Contrary to your claim, this doesn't increase the price by 30% because the price is not determined by the software company but by customers' willingness to pay.

Price of software on iPhones is impossibly low. That's because for every piece of software, there's a lot of competitors providing such software and ecosystem evolved toward incredibly low prices.

That's also why making make mobile software is a terrible business for most.