If I recall, OSHA requires any potential fall height greater than 4'-6' (depending on circumstance) to be protected. Workers could use means like harnesses with tie-offs, which is probably what they have been doing. But arguably a railing or parapet wall is safer in many cases because you help reduce the human error factor of somebody not knowing how to tie-off properly. Assuming the railings have toe-boards it also reduces the chance of objects rolling off the edge onto people below.