index scan through a subquery

Why should these queries have different plans?

create table foo (a int PRIMARY KEY);

Q1: explain select max(a) from foo

> Result (cost=0.04..0.05 rows=1 width=0)
> InitPlan
> -> Limit (cost=0.00..0.04 rows=1 width=4)
> -> Index Scan Backward using foo_pkey on foo
> (cost=0.00..76.10 rows=2140 width=4)
> Filter: (a IS NOT NULL)

Q2: explain select max(a) from (select * from foo) as f

> Aggregate (cost=36.75..36.76 rows=1 width=4)
> -> Seq Scan on foo (cost=0.00..31.40 rows=2140 width=4)

I need the lovely index scan, but my table is hidden behind a view, and
all I get is the ugly sequential scan. Any ideas on how to convince the
optimizer to unfold the subquery properly?

Bill