Freeing data is problematic for privacy reasons.

Even if you don't have problems with releasing your own medical data some other people have problems. Anything that might reduce in some people refusing to sign up for the service reduces the virality of the internet service. Successful web 2.0 sites need high virality.

You could add an optional button to release health information to the public but I would still imagine that it would be hard to convince a sizeable number of users to switch to that option. Users are selfish ;)