I think the difference is:

iPhoto says "This is a face. Would you like to type in who it is?" And Alice would type in "Bob".

Whereas Facebook uses Bob's profile information to say "Is this a photo of Bob?", leaking Bob's identity to Alice.

From my understanding the distinction is whether or not personal data is being saved, so as long as iOS isn't saving/classifying specific 'faces' it's not illegal.

Sounds like there's a difference between gathering prints (faceprints, fingerprints, voiceprints) that can be used to identify people v. simply identifying that something is a face, a fingertip, or a voice, which is what iPhoto did.