Si. they use fixed cogs for a few reasons. one is that they do not need brakes on the bikes, which makes them more low-maintenance, and saves a bit of weight.
there seems to be a fixed-cog culture among couriers.. pedalling all the time should get you places faster really, as you can't simply coast along... i'd like to try a fixie, but i'd probably die, or at least kill my shins big-time.