I can see manual app review getting costly, but Apple somehow doesn't wage war against free-to-user apps, nor they put limits or fees on excessive app releases. They're only upset when someone else makes money without giving them a cut.This is exactly where most of the cost of operating the app store comes from. I'm not going to comment on how much of Apple's fees go to paying this, but testing software to release standards is expensive.
I remember 20 years ago, just to get started on a PS2 title, you had to pay sony a crazy dev fee just to get your SLUS code (I remember someone telling me it was about USD$50k). Then all of the devkits cost about USD$20k, and then sony still took a big chunk of your sales price.
How exactly would you like Apple to handle free apps, which still need to be tested and vetted? What if there was a $10k dev charge for each round of testing? How many free apps do you think there would be in the app store? Its true - all of the paid apps are subsidizing the free apps in the app store. If Apple didn't squeeze out other payment processors, every paid app would turn free, they'd use Square or something to sell people in-app purchases instead, and then the whole ecosystem would fall apart.
I guess another question we can ask is what are they testing for? Sony was exacting - there were strict rules about stability and content that had to be met. Your title couldn't stall on a waiting screen for longer than a period of time (I remember having to optimize our start-up on one title to meet this requirement). Even the terminology you used had to be correct - there was a list of acceptable names you could use to refer to the console parts like the controller - like you could call it "PlayStation 2 Dualshock Controller" but not "PS2 Dualshock" and stuff like that.
On the one hand, Apple isn't as exacting, but on the other hand presumably they're testing for other things you wouldn't have to worry about on a PS2 like calling dlopen on forbidden dylibs or sending the addressbook contents to a server. I'm sure that a lot of this is automated, but some of it will require a human to interact with.