Google, Facebook, and Amazon are all reaching the point where they dominate their business space, and can manipulate that space for their benefit. They really are large enough that an antitrust investigation is warranted...large companies don't necessarily have to be broken up, but if they are manipulating the system for their benefit or to the detriment of others it's a problem.
Apple is a bit different...it competes with its own closed systems against other companies. It's a lot closer to how car companies work, really. Let's call Apple BMW...they charge for all kinds of additions to their cars, and people pay for them. It wouldn't be considered an antitrust issue, because you can go buy a car from Honda instead if you don't want to pay for those additions. That's how all of its stuff works...you have to choose to live in the Apple ecosystem, you don't have to, and then you have to pay to stay within that (as you do with other systems from Android, Windows, etc). It's not totally closed...you can use whatever earphones you want with your iPhone still, even if it requires an adaptor. Choices in what to offer are Apple's to make, and people can always choose to leave (yes, it's painful, but you can move to other platforms, making it easy or free is not required).
The App Store is a different issue...if you want to put software on an iPhone you have to go through Apple. But, pretty much everyone else has the same setup, and charges similar rates, Apple really isn't out of line with the industry here. And, the App Store isn't the only store...working within the store means accepting rules, just like within any other store. There is competition between the stores, there are some apps that don't exist in the Apple area (and lots of viruses, don't forget that!), so the problem here is people want things they can't have or developers want a bigger slice of the pie instead of sharing with Apple. Those aren't really antitrust issues, it's complaints that Apple does things people aren't happy with, not that they are doing anything illegal. (It's important to realize that the App Store isn't a industry in itself)
So, the question is more if this policy is correct...it's not illegal, but should there be a limit on how much of other people's effort a company should benefit from? Generalizing that, almost all of us work for someone else...should Jeff Bezos benefit so much from the work of people making low incomes? What percentage of the money made by independents that sell through Amazon go to Amazon? Does anything stop them from changing their charges? And, do these companies do things that kill competition? The last may be something Apple does through the App Store, and they shouldn't be. The real hard problem is how to deal with all of this legally...a lot of this comes from unregulated capitalism, which is just like what happened a century or so ago in the US in industry. There aren't limits to any of this, how those limits are set is really not a simple problem to solve, but it's obviously necessary now.
