Seqscan in MAX(index_column)

(Perhaps a newbie question, but I tried to google this out without success).

Why postgres does an expensive seqscan to find the max(value) for an indexed
column? I think MAX() does not know or cares if a column is indexed, but...
Should not it? BTW, is there some smarter trick to do that?

I know I can just do a very fast (SELECT pk FROM foo ORDER BY pk DESC LIMIT
1) instead, but my coleagues are arguing that MAX(indexed_column) seems to
be a lot
more smarter in MS-SQLServer and I end up without a good response.

Thank you,
--
Paulo Scardine
Brazil