To turn the tables...Why do ATC consider ack'd flight levels optional?
If we have filed a flight plan, which has been accepted and acknowledged with a stated cruise flight level, how do ATC justify holding us down at a lower level?
On more than one occasion, I have uplifted OPS-minumum fuel for the filed and accepted flight plan, then been held down below the planned flight level for a significant portion of the cruise meaning that we risk not having enough fuel for the flight. The answer is not - "you should have uplifted more fuel" - we fly to extremely tight margins and if fuel is more expensive at departure than arrival, we uplift the OPS minimums.
The whole point of OPS fuel regulation is that it should cover all forecasted eventualities with sufficient safety margin for safety - so how do ATC justify throwing a completely unforecasted spanner into the equation by holding us down below the planned flight level?