I would guess that it doesn't really mean skid in a riding situation. That would be silly, and laws are never sill... oh, wait.
In the UK, a fixed wheel counts as "1 brake" so legally you can ride fixed with only a front brake.
Having a rear brake is only a disadvantage if you flip your wheel and the two sprockets are different enough in size that the blocks won't work both ways. Why not just have a brake and choose not to use it?