"<query expression body> ::= <joined table>" grammar rule not accepted by Postgres

This expression is (I think) a well-formed query, but is rejected by
Postgresql 8.4 (t is a table name).

t t1 NATURAL JOIN t t2;

Here is the relevant parts of grammar (with some optional elements omitted
for simplicity):

<query expression> ::= <query expression body>

<query expression body> ::= <joined table>

<joined table> ::= <natural join>

<natural join> ::= <table reference> NATURAL JOIN <table primary>

<table reference> ::= <table primary>

<table primary> ::= <table or query name> [ <correlation name> ]

<table or query name> ::= <table name>

Nor is this use of <joined table> accepted in the <query expression> form
of a <with list element>:

WITH tw AS ( t t1 NATURAL JOIN t t2 ) TABLE tw;

This leads me to believe that the unrecognized grammar rule (in SQL99
standard terms) is

<query expression body> ::= <joined table>

There are, of course, other ways to express the operation, including
SELECT * FROM (t t1 NATURAL JOIN t t2);
But I am curious why the first expression is not accepted, besides the
immediately obvious "it's not in the Postgres version of the SQL grammar."

Clem Dickey