...they assume that the outside surface of the riser is parallel to the travel of the string during a shot.
Allen, I think I understand what you're saying -- but, actually, whether the riser is parallel or not has little to do with how the laser actually operates.
The riser itself serves as nothing more than a solid mounting point which remains static during the laser's operation. By adjusting the laser via the threaded rod out until the beam comes in contact with the string, you can then make sure that the laser head is pivoting "square" by running the beam itself up and down the length of your string.
Once you're satisfied that the beam is contacting the string properly while pivoting it up and down its length, it's a simple matter to nock an arrow and pivot the laser head down the entire length of the shaft to ensure the laser beam follows along the top of it. If you get the laser out to the tip of the arrow shaft and the beam is still not sitting exactly in the center of the shaft, you'll know you need to adjust your rest's centershot in or out to accomodate.