In WA the driver and passengers drinking in a car are considered to be 'Drinking in public'... I know this because my old boy got done for having passengers drinking in the car (yes, even though we wasn't drinking he still got fined because a car can't be driven with any alcohol being consumed).
It all seems quite harmless, but so does having a beer in the park (also illegal), most of us would be pretty tame and enjoy our beer but there are those idiots who F%$K it up by throwing bottles out of the car, annoying the driver, yelling at people in other cars and pedestrians. As for the people on push-bikes and skateboards, the law is there to protect them (and I'm sure the fine would be hefty enough along with suspension of license (if they have one))...even though you can't smash through someones house on your skateboard, you could very well skate infront of traffic, cause a car to swerve which then causes some major damage etc.
If you don't want to get done, don't do it...or make sure everyone keeps an eye out for cops and don't take a swig when they are driving right beside you
One of the downsides of living in a 1st world countries! Damn those laws!