PDT but not WEST

I am baffled.  Both PDT and WEST appear as valid timezone abbreviations, and each have unique values, but:

test=# select timestamp with time zone '2011-09-29 18:00 PDT';
      timestamptz
------------------------
 2011-09-29 18:00:00-07
(1 row)

test=# select timestamp with time zone '2011-09-29 18:00 WEST';
ERROR:  invalid input syntax for type timestamp with time zone: "2011-09-29 18:00 WEST"
LINE 1: select timestamp with time zone '2011-09-29 18:00 WEST';

What am I missing?  Is the parser insisting on three-letter time zone abbreviations?  Should it be?
--
-- Christophe Pettus
   xof@thebuild.com