Index build temp files

Greetings,
 We were surprised recently to note that the temp files that are created during a CREATE INDEX don't go into either a
temptablespace set for the user or into the tablespace which the CREATE INDEX specifies.  Instead, they go only into
base/pgsql_tmp/. This doesn't allow for any flexibility in defining where to create these potentially quite large sets
offiles.
 
 Shouldn't these temp files be going into the temp tablespace for the user creating the index instead..?  Or perhaps
intothe tablespace which the index is being created in?
 
     Thanks,
    Stephen