Apple's change doesn't make tracking opt-in or opt-out, it forces users to make a choice, with both choices being equally easy to make. It's a perfect compromise between default privacy and default tracking -- presenting the user with an unskipable dialog that makes sure that neither option is buried in a setting someplace, that presents both options on equal terms.
So Facebook wants to argue that it's a worse experience for consumers if they're not being tracked, but the truth is that they know a large portion of users don't want the experience that Facebook is offering. That's why Facebook is scared. Not because Apple is bullying users into being private, not because (in this case) they're attacking the Open web, but because Facebook knows that when users are presented with a clear choice that makes it equally easy to opt into or out of tracking, they choose to opt out.
Advertisers are always saying that people want what they're offering. But they are so stinking scared about being asked to prove it.