Multicolumn Primary Key

We've got a table that has a definition as follows:

CREATE TABLE linking_table (
fk int8 REFERENCES source_table( pk1 ),
value int8,
PRIMARY KEY( fk1, value )
);

I would've thought that the multicolumn primary key would behave as a
multicolumn index is supposed to behave per

http://www.postgresql.org/docs/7.4/static/indexes-multicolumn.html

where the behavior of the index cascades from the left rightward across
any columns specified in WHERE.

But a query like

SELECT COUNT( * ) FROM linking_table WHERE fk = '42';

yields a sequential scan.

If I add an index to fk, then the same query yields an index scan, as I
would expect. Is this because, according to the docs, a primary key "is
merely a combination of UNIQUE and NOT NULL"?

If so, then why do primary keys afford index scans of single columns
specified as primary keys?

This is in postgres 7.4.5, btw.

-tfo