"Officer, I wouldn't dream of obstructing your investigation in any way. Tell ya what, I'm just going to go waaaaayyyy over there and keep my mouth shut so I don't get in the way."
In other words... non-cooperation isn't obstruction.
A tree in the middle of the road is an obstruction. A road that goes around the tree instead of straight when you want it to isn't an obstruction; the road just isn't cooperating.
I wonder, if the officer stopped a bus for speeding or for running a red light, is he going to detain every passenger on the bus and demand of them their identity??