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(at)thebuild(dot)com