Because CDT isn't rational. You don't always have to act only for the sake of things that you can cause. If you're a transparent agent then you sometimes have to become the kind of agent that will carry out a precommitment. If that commitment fails then the rational thing to do is to carry out your threat.

EDIT: No-one else in the thread appears to understand that you don't need to have an additional reason (like a third party agreement) in order to carry out your threat.