ZD,
The limo is probably not much more than a taxi, is safer, and the boys enjoy it more.
Walking is not an option, the houses they are going to are too far apart. Time is also limited, because most homes are not approachable after ten o’clock or so. This is actually one of the most efficient ways of raising funds, because the boys take nothing, the only expense is the car (some rent costumes, but I don’t think this group is).