It's because it leaves them at risk of using previous precise answers to others as precedents for future cases. Being opaque allows them to not be too committed, and outside of situations like "but you accepted XYZ app which does exactly the same, I don't understand"
You can still claim XYZ app does what you did and didn't get punished by that, but they can never admit it in paper/words.
Scammers will try to skirt rules by making sure to adhere to the letter of the law, even if it’s plainly obvious they are violating the intent of the rule in question.
Rule 407b: no real money gambling apps
Scammers: it’s not real money. It’s Linden Dollars. Or our company script. Or our new NFTs. You can’t reject us.
Should the rule have to list out every single thing that could possibly used as currency? Because there are people who will argue that point.
The more explicit you are with the rules, the more specific bad actors will try to weave around them. It sucks because it means the scalable, machine-assisted review process comes up with more false positives, but it's a two sided market and the marketplace owners care more about the users than the devs.
I feel like the U.S. tax code is a good example of when you try to go more explicit with the rules.
What you're referring to is the application of power, and their retention of arbitrary power, specifically.
Ethics is if they used that power for abuse or an unethical goal, or to acquire an even larger amount of arbitrary power.
Pragmatically, it's their app store, and they need to retain a non-trivial amount of power to police it. It's a requirement of ownership. If they don't do so, it will devolve to an even worse garbage dump, and rather quickly.
We're of course going to complain about inexplicable rulings, things we don't agree with, etc. but that doesn't change the equation unless they get so obnoxious that other places are more attractive or it violates some law.
For the one in 100k poor guy who got on the bad side of the AI this seems like a raw deal, but if you tweaked the system to avoid it, you'd probably end up with something 5 times as expensive and 10 times as slow. Queues of weeks to get your app approved kind of slow.