I was totally unaware crew had drinks on the bus at Long Haul, over here at regional gezz you would be lined up in front of a firing squad if you even thought about doing that or even pulling a packet of smokes out of your bag in public so I can see where the different views on this matter arise.
Not having a go at anyone and if I could I would be the first one having a beer on the bus/taxi etc, however doesnt it state somewhere you cant consume alcohol in uniform (which you have explained you are not), on duty and on company property. Wouldnt the bus still be regarded as company property? They are afterall paying for it.
Not taking any sides here just interested, because if cabin crew management wanted a reason to take action against crew this would be handed to them on a plate.