I think you're misunderstanding. I'm not proposing that Facebook be _forced_ to allow third-party access, which would carry the risks you've pointed out. I'm simply proposing the federal government not aid them in their market dominance.

Facebook could still prevent third-party applications from functioning, but it would have to do so by methods inside of the HTTP specification rather than legal ones. For example, it could simply ban users who authenticate via a third-party app.