John Connolly -
As a practical matter, Shinnecock is a summer season club with a relatively small membership that likely lives most of the year in & around New York City. As I recall, the only reason they decided to host the US Open was that the USGA said they would take care of pretty much everything involved with organizing and staging the tournament. That includes finding volunteers to work the event, be marshalls, arranging for parking, transportation, housing, etc.
The burden of organizing and staging USGA championships typically falls on the membership of the club hosting the event.
DT