I was under the impression that when a customer signs an ACMI the carrier is basically "on demand". If the economy heads south, as it seems to be doing world wide the on demand work will dry up no matter who picks up the tab. No customer would want to fly around an aircraft empty. I could be wrong but I don't think so. I cannot imagine a carrier not having some sort of an "out clause" or some other instrument to protect them.
As I understand basic ACMI contracts, the customer guarantees payment for a minimum number of block hours per month, even if they are not used. The customer pays the contracted price for any hours used over that minimum. In return, the customer has full scheduling authority for the airplane, within the ACMI provider's legal constraints. So, if the customer has no load to send, he can keep the airplane on the ground, therefore not incurring the fuel cost.
As for any "out clause," I suspect those are very dependent on the individual customer and contract.
In context, I have no clue how AAWH and DHL constructed their "block space agreement" to address the times when DHL decides they don't want to pay for the airplane to move. IF it is constructed such that AAWH/PAWW is obligated to fly the "scheduled service" run anyhow if there is other load, then it may have an element of "scheduled service" to it. OTOH, IF DHL controls the schedule and can tell other freight forwarders/shippers that the airplane will not make a particular run, then it appears to be strict ACMI.