Back pack waist straps generally serve two purposes.

1. When back packing, the waist strap allows you to shift much of the pack weight off your shoulders and onto your hips. Giving your shoulders a rest.

2. When mountain biking and you take a tumble, it keeps the pack on you rather than bouncing up against your head.

If it bothers you when road riding, just don't use the waist strap. I bend mine around the bottom back side of the pack and connect the buckle, so it stays out of my way.