The users wouldn't need to decide whether it's reasonable, they could just ignore it. Not allowing information to get to the user is a different thing though.
At the end of the day there are costs associated with doing business online. If Facebook was able to have their own payment processing pipeline, and still charged no cut, that's them processing payments at a loss, and subsidizing it with other income streams.
The Apple cut doesn't just cover payment processing though, (in Apple's opinion at least) it also covers the costs of maintaining the app store, reviewing apps, developer tooling etc.
There's really two separate questions here, is 30% as a cut for the entire App Store ecosystem reasonable, and is it reasonable to charge a cut paying for the entire ecosystem to someone who really is only "getting" the payment processing parts of it.
In a sense, when Facebook is providing this kind of platform, they're essentially competing with the App Store. Instead of whomever is running "Jasper's Cooking Class" making a cooking class streaming app and posting it to the App Store platform, they're able to use the Facebook platform to provide the same service. Because this becomes sort of a platform-in-a-platform, it starts getting expensive pretty quickly, because you have to pay for both.
Of course, the people behind "Jasper's Cooking Class" (presumably Jasper himself) are probably not in a position to make an app and posting it to the App Store, so Facebook providing a lower barrier to entry is probably a good thing for the market as a whole, but they're not doing it out of the good of their hearts, it's still a very deliberate business decision, aimed at keeping people on Facebook, where the users are monetized in other ways.
I understand Apple's position in this case. Yearly subscription is just a symbolic payment, and, frankly, I don't think it covers the costs of Apple's employees who spent their time verifying my org's account. So what is left are the fees from app sales themselves.
But what about apps that are free, or pretend to be free like Facebook? Their creators still find ways to make money, but in a way that evade's Apple's fee system. Why should FB and others in a better position that traditional app developers? Both groups use the same infrastructure and both should pay similar fees (although frankly, these fees are way too high, and I think everybody except Apple agrees on that).
Do you think the landlord should be allowed to make them remove the sign?
If it's accurate, what's the harm in the public knowing about it? If it's a two square meters stand in a food court, the public might find the landlord to be insane. If it's a five thousand square meter super market, I'm sure the public will find that appropriate.
Keeping the Apple tax secret suggests that Apple believes the public wouldn't find them appropriate, either because "the public doesn't understand" (easy to fix by writing a blog post) or because they aren't appropriate (given Apple's profit margins, I lean towards this).
Also, I find the idea that a blog post is an easy fix for the public not understanding what the developers are paying for with Apple's 30% cut to be a huge simplification. At the very least a major ad campaign would probably be necessary, and even that's probably only gonna reach a subset of their users.
Sure, a single blog post might not reach everybody. But at the same time, lots of people won't care, and everybody that searches for "Apple Tax 30%" on the web will find their blog post. Maybe they can get Bono to write a song about it! ;)
I found https://news.ycombinator.com/item?id=24302003 informative on the topic.
I don't think that Apple is arguing that e.g. Facebook wouldn't be allowed to say on their website how much commission they pay to Apple. Apple also says quite openly that and how much they take [1]: Developers earn 70% of sales from in-app purchases and Apple collects a 30% commission. That page actually looks like the blog post I mentioned: explaining to users why Apple's share is appropriate.
[1] https://www.apple.com/ios/app-store/principles-practices/