Why are some legs of a SID considered transitions, even when there's only one?
After some digging and exploratory route planning in ForeFlight, it seems I've arrived at an answer that satisfies me: the last waypoint is listed as a transition only if the departure officially ends prior to it, and flying to the transition is optional. If there are other (more correct or plausible) answers, I'd still love to hear them.

The text description of each plate describes where the SID ends. In the case of BAYPO8, the last instruction is on-track to BAYPO. In the case of ENDED8, the text describes on-track to LACEN. So, it would seem that flying to LACEN is required when assigned the ENDED8 departure, however flying to CAMJO is optional for the BAYPO8, because you can drop off the procedure after BAYPO.

This is supported by the route planning features of ForeFlight. When I selected BAYPO8, and then "No Transition," my route of flight for the departure ended at BAYPO. I also had the option to choose the CAMJO transition. This is also supported by analysis of other similar departures that include only one final waypoint option.