I'm betting that Good Sam is unwilling to accept the liability attached to just leaving something in a parking lot. Turning it over to a live person means their responsibility for it is over. And since you couldn't be there, shop owner/employee is the next best thing. Not the same thing as you calling a tow truck and arranging delivery to a closed facility. In that case, YOU are the responsible party.
This incident does point up a rather surprising benefit of a membership in this service- coverage for the vehicle no matter who's driving it/ involved in the breakdown/accident. Triple A (what I have) coverage is strictly for named parties. They won't tow a rig AT ALL unless the covered person is there- and at both ends.