What could we call a "market for predicting decisions"?
If I understand the question correctly, you're asking for a word that describes a market where investors speculate on the future decision of a single agent, rather than on the net effects of many decisions made by many agents.
I'd suggest preemptive market. You're attempting to preempt the actions of a single agent.
May I suggest Prospect Market ? Participants evaluate the current prospects of a future event.