Would you be ok if they did it that way? A warning that says "by using Facebook, you are agreeing to be tracked", at which point the user could either continue or uninstall the app?
The only thing Apple is preventing them from doing is making an even larger shit ton of money by essentially knowing every app you ever use for anything that includes the Facebook SDK.
Note the same thing is true of Google. Google can (and did, and does) make shit tons of money by showing you targeted, relevant ads just based on your search keywords, or alternatively on a partner site by showing you ads just based on that site's content. But no, they need to make even more gargantuan shit tons of money by tracking your every move in Chrome and Android.
“If consent is bundled up as a non-negotiable part of terms and conditions it is presumed not to have been freely given.“
That people won't opt into something that they didn't ask for and isn't primarily for their benefit is unsurprising.
That's like saying if you put a price on something, it will kill that thing. No, it doesn't.
I opt-in to using my car to drive, and I opt-in to using my laptop to write code.
I don't opt-in for tracking, advertising, or other negative externalities.
And I don't even think it has to be the primary feature. As someone else mentioned about maps, map apps are still plenty useful without location tracking (indeed, analog maps worked well for millennia without it), but I know if I opt in to location tracking while using a map app that I'm going to get a lot of additional useful features, so I do it.