150% may be plenty to encourage enough pilots to pick up the uncrewed trips. The problem is that since it is not provided for in the contract it is not enforceable. The company could pay some, but not others, and there would be no recourse. The union (APA) wants a letter of agreement to cover the remedy so that it will be enforceable if it isn't applied uniformly.
At my airline we have a provision for the company to designate premium pay for specific open trips. We can see in advance if the trip will pay 100%, 150%, or 200% of the normal rate and make our decisions accordingly. The contract defined the conditions and requirements on both sides so there are no surprises. The reports from our union's scheduling committee is that the company is quite happy with the results of this process when they decide to designate premium pay trips. AA and APA could sit down and come up with such an agreement in a few hours but, so far, the airline hasn't been willing to do so.