The squirrel-mode for trip scheduling is pretty standard for many operations, but not getting compensated for time being jerked around is not (or should not be, anyway.) That kind of on-call availability demands compensation, whether a full-time salary, or some kind of trip cancellation policy for contract work. If these folks aren't willing to pay adequately for your time spent with no-go or cancelled trips, then it's time to walk. But as said above, do the walking in a professional manner.