But, This isn't yet a stretch into all ad spend - I do think Apple will go there too eventually.
In practice Apple is saying paying to boost an organic post is in effect an "in app purchase". They are correct. The purchase happens in the Facebook "blue" app. I actually haven't seen or done it with a boosted post but that is what Apple is saying. You post something via the Facebook "blue" app...get a notice that it's been seen by a few people, so pay a bit to get it seen by more and all of that is done via that one app....so an "in app purchse".
Regular ads are done with the Ads Manager app....a separate app from the Facebook "blue" app. So, ads themselves are not yet subject to this 30% bounty. Again I haven't done it on the app but in the web ads manager you can boost a post from there too so likely you can in the Ads Manager app...not sure though. If you can then you avoid the 30%. I suspect most if not all small businesses, etc have no idea what Ads Manager is or are not very familiar with it.
I have a particular disdain for the app store generally so I don't like this at all. But, you can see where Apple has been and will go...force everything to an app instead of mobile web browser (by nerfing webkit on iOS) than force 30% tribute on "purchases".
I have no idea how this escapes any anti trust scrutiny, especially compared to what M$FT did with IE back in the day and got dinged for it.