Paul,
Very good question, but (yes there has to be right?)
When you use the term "default". By "default" SQL Server tries to create a PK as a clustered index. There already being an index on the table which is clustered is not necessarily a default action.
Otherwise, I enjoy your questions. They are thought provoking and good teaching points.
Steve Jimmo
Sr DBA
“If we ever forget that we are One Nation Under God, then we will be a Nation gone under." - Ronald Reagan