It's fully legal, it's called UVP.

You state the price it "freely" cost on your product.

And if it cost somewhere more because additional costs is clearly visible.

Lastly this is not about saying "you can buy it cheaper there" but saying "apple gets 30% of this". Which the difference between not so nice marketing and making prices more transparent the later one is a fundamental part of a (working) free marked...

Printing the manufacturer’s suggested price on the outside of grocery packaging is common.

No. But you should expect (I think?) the grocery store not to ban the cookie maker from packing notes about how to order from the maker directly. (Edit: I mentioned that because, per the story, I think Apple has a policy of prohibiting you from telling users how to circumvent Apple's cut.)

I actually don't know if grocery stores generally require this -- maybe they do. I suspect it's not an issue because it's generally going to be more expensive to do one-off orders directly from the OEM, unless it's an order too large for the grocery store to want to manage anyway.

Edit 2: Earlier comment where I made your point in a different context: https://news.ycombinator.com/item?id=23162403

I think this would be great, yes.

The store has no reason to hide the markup if its fair, reflecting the convenience they added.

Well, there's a reason Arizona Tea prints "99c" on the can

It’s a known practice.

https://youtube.com/watch?v=hWObybWWGW4

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

I don't think your example is right, it will be more, if you buy this box of cookie, Visa will take 25 cents + 3%, the grocery bank will take 2%, etc. Here Apple enforce that you do not have to create an account with Facebook to purchase goods and services on Facebook. That people can use their Apple ID, then that's a 30% cut.

By the sounds of it Facebook’s note didn’t say “buy from us”, it simply notified users that Apple will take a 30% cut.

I’m loathe to care either way here because I don’t think any giant multinational needs me to speak in their favour, but the part that grinds with me is that Apple blocked it under a rule about “irrelevant information”. Given that FB created this feature for community events the fact that 30% wouldn’t be going to the host of the event feels like very relevant information to me. If Apple wants to have a rule that says “we decide what you can and cannot say” then they should just make one rather than hide behind other rules.

Should such a note be illegal?

You won't be able to. Minimum volume orders are a thing.