subquery question

I've dug around on the Postgres lists for a bit and I haven't found a 
good explanation of why this query is not allowed. 

SELECT Temp.team_id, Temp.count_agents
FROM (SELECT TR.team_id, COUNT(TR.agent_id) AS count_agents
      FROM teamrel TR
      GROUP BY TR.team_id ) AS Temp
WHERE Temp.count_agents = (SELECT MIN (Temp.count_agents) FROM Temp)

I know that column aliases are not accessible in the WHERE clause, but why can't we access outer table aliases in a subquery FROM clause?  Is the table alias rewritten into the select clause?

-Tom