The point is Apple forbids developers from letting customers aware of the 30% tax.
And if you really want to oversimplify the discussion to a brick and mortar analogy, it's common for grocery stores to make taxes explicit in receipts: https://i.imgur.com/Lp3nwlH.png
That’s not the same thing at all. I’m saying that you bought the product from a grocery store, and the product itself contains information discouraging you from continuing to buy from the grocery store.
In the app case, you bought an IAP from Apple (I know it’s Facebook’s app, but that doesn’t change the fact that the transaction was executed by Apple) but somehow think it’s ok for there to be a note saying you can pay less if you buy it on your computer instead.
Facebook want saying they could pay less. They were saying, if you pay $10, $7 goes to the event, $3 goes to apple.
It didn't say "go to this other website and just pay $7". I think it's totally reasonable to disclose the fees, I would even argue that it's the morally correct thing to do.
And if you really want to oversimplify the discussion to a brick and mortar analogy, it's common for grocery stores to make taxes explicit in receipts: https://i.imgur.com/Lp3nwlH.png