This is driven by the 'service delivery' fools. To them safety is of no importance, and furthermore they have no jurisdiction over the flight deck crews.
I go to the bus when I am ready, and have briefed the crew as required by the OM-A; if that is 'late' in their interpretation, so be-it. I will not answer to cabin crew personnel, unless they can tell me why I should explain how I can be late when I am not on duty until STD -1hr, which is well after the bus departure time.
That's my argument, but I am sure many will disagree.