In all of the notices, Apple is usually quite explicit in what the problem is, including attaching screengrabs, and they will respond, if I ask them for further clarification.
Over the years, I've had over twenty apps in the store, but most are retired.
I'm down to seven: https://littlegreenviper.com/AppDocs/
In a couple of cases, they actually found crashes that I missed in my testing.
Trademark use.
I have had apps rejected because I used a trademark (usually Apple's) in my app name or description.
For example, I had submitted an app called "Bluetooth 8-Ball for tvOS".
I was told that "tvOS" was not allowable. I had to use "TV".
This kind of thing has happened a few times. I casually use Apple trademarked terminology a lot, but I can't be as sanguine about it when I submit apps.
Full Disclosure: They also had a problem with the app being a "demonstrator" app. They don't want us releasing apps that they don't think we're "serious" about. They had a point, and I ended up withdrawing the app.
They sure aren't looking at developer account fees to hold their bottom line up.
It's low enough that I can easily keep two organizational accounts going.
If the goal is to prevent piracy, well, as with other forms of DRM I as a paying customer don't appreciate being treated like a thief. Dedicated pirates can and do just buy stolen enterprise certs on the black market anyway.
I don't think that's their goal.
I suspect that it's all about "brand reinforcement."
Apple is (arguably) the world's most valuable brand. Those don't come in Cracker Jack boxes.
They don't want some knucklehead running around, showing some crapplet that makes the brand look bad, and they certainly don't want them installing said crapplet on their friends' phones, so there's a bunch of folks running around, making them look bad.
This makes that a lot less likely. If they restrict it to paid accounts, then they have an assumption that the people writing the apps are "serious" about developing decent software.
I suspect that a big part of them buying up TestFlight was because they didn't want a company out there, making it easy to install un-vetted crapplets into a wide range of devices (which the old TestFlight allowed).
I have some experience with this. I used to work for a world-renowned corporation that made photographic equipment. Their brand is right up there, with Apple.
They would go nuts about sample photos getting out of the company. It was really difficult to report bugs, or even share test results, because the sample photos couldn't make our cameras look bad.
There's a great deal of controversy about Apple's iron-fisted control issues, but I do understand. I'm not always happy about it, but you can't argue with the results.
Basically, I support Apple’s “walled garden.” It’s a pain to deal with, but it makes their App Store a lot more valuable.
There is a lot of loud, vocal, opposition to it, but the vast majority of regular (as in non-geek) people like it just fine.
The fact that they took the time to explain and justify the rejection made all of the difference. I have been on the receiving end with Google and it's much more difficult to deal with.