The design philosophy behind this rule becomes apparent when reading the description for Bow Levy in the Meshwesh troop types.
This makes some sense, but what do you actually do with them on the tabletop?

Sure, they're cheap - but they can't really fight and have no ranged attack. Sure, they don't suffer combat-related penalties for being in difficult terrain - but their movement through such terrain will drain command points, and their movement rate is already very slow. Consequently they don't appear to combine well with Light Foot and Skirmishers as a 'rough terrain force'.
So . . . what do you do with them? Placing them in open terrain seems hazardous to their health against an array of possible opponents, and 'hiding' them in difficult terrain seems somewhat pointless.

Cheers