If they don't control the Twitter clients, they can't avoid the clients from using multiple platforms (e.g. Facebook, app.net, the newer platform). Then clients can switch away users from Twitter seamlessly.

This is pretty clear from their API policies.

This makes me think that all this locking is not about monetization. It's about protecting their user base.

Facebook is following the same direction. Their current api version does not allow to build an alternative Facebook client.