Even if it was the User's data and not FB's (which is argued downthread a bit...) there's nothing that requires Facebook to make it accessible to other companie

Even if it was the User's data and not FB's (which is argued downthread a bit...) there's nothing that requires Facebook to make it accessible to other companies. There is real cost in creating an API, running it, managing it, etc. One could also argue that the User already has all their own data, they just didn't collect it very well. So really they're trying to use FB as their data collection platform - which FB is free to do or not, as they choose.