Let's say I put all of my friends' birthdays in my calendar. Now the data has left the silo and I can reshare it with whoever I want. Just how is anyone going to revoke that ability?
Also, it's not meant to be a complete solution by itself. It's meant to be a tool supported by other legislation. So it would totally be possible for Facecorp to crawl the web and put together a non-revokable database of private individual's information, outside of Trusted Execution engines. But to do so, they have to go to that effort, instead of just sweeping user data.