When I first heard it, I thought it was a very big new that finally apple allowed to sign the apps for their devices without $100. Now I don't know why, I see almost no one is using it.
Last time I checked the "free" provisioning profiles it generates are valid for something like 7 days. It's not really convenient for anything but experimenting with iOS development.
Plus you still can't do it if you don't have a Mac.
I don't know the license terms, but I can imagine that there are some terms that disallow people to distribute apps for users to self-sign them on the web.