It's not about the money they're bringing in. It's about the limits they impose. iOS users can't buy apps except through Apple, which means they are effectively gatekeepers for more than half of U.S. cell phone users. And Apple has used those limits to forcing third parties that make apps available through their App Store to take payments through Apple, thus allowing Apple's similar services to unfairly compete (because Apple pays maybe 3% for their credit card transaction fees, while Spotify likely pays 30%).
OK, so I hear this all the time but let's take a step back for a sec. I used to have a small software company that maintained its own online store which required hosting (back then this flip-flopped between colocation and a local high-speed connection, now I'd just use a $20-$30/month VPS) and my own time maintaining site code/security/PCI conformance testing (let's call that 20 hours/month for a small business) - budget about $1,000/month overhead for running your own e-commerce site and somewhat less if you use an off-the-shelf solution. Credit card processing fees for a small business are usually a small per-transaction fee (I've seen up to $1) + 3-4% of the order depending on the card. Advertising/search optimization will add more but you can choose how much to spend and there are "free" organic methods of getting the word out as well.
For a modestly successful app developer selling 5,000 copies of a $100 app in a year, you have $500,000 in revenue less $25,000 in credit card processing fees (worst case) and $12,000 in hosting costs for $37,000 in e-commerce expense (7.4%). Add a $24,000/year marketing budget and you have a total CoGS is $61,000 (12%) which translates into a gross profit of $439,000. The $100 price is arbitrary but you want to keep it above $50, otherwise you end up with lots of credit card fraud (I saw something like 1% of my sales get charged back until I raised the prices above $50).
The same app developer on the Apple App Store will pay a 15% commission on the $500,000 revenue ($75,000) and you'll likely still want to spend a little on marketing (let's say $12,000/year) for a total CoGS of $87,000 (~17%) and a gross profit of $413,000. But through the App Store you *don't* have to worry about chargebacks (people *can* cancel an order but it is harder to get the credit card company to cancel an App Store charge) so you might decide to lower the price or provide in-app purchases, etc. to *increase* your sales, offsetting the difference in CoGS. Oh, and you don't have to deal with e-commerce site issues, people having trouble getting their card accepted on your site, etc.
So as a small software developer, I am 100% OK with paying Apple's App Store "tax" for the convenience, flexibility, and security it offers. I wouldn't complain if they lowered it (10%?) but I also know they are running a very large App Store that handles far more traffic than my little private online storefront would ever see in my lifetime.