Incorrect Query Plan With Partitioned View On SQL 2000

I have a partitioned view containing 4 tables (example follows at end)

The query plan generated on a select correctly accesses just one of the tables

The query plan generated on an update always accesses all four of the tables. I thought that it should only access the partition required to satisfy the update. Can anyone please advise whether:
a) Is this is expected behaviour?
b) Is the partitioned view incorrectly configured in some way?
c) Is there is a known bug in this area

Note that the behaviour is the same with SP1 on SQL2000

I would be very grateful for any advice

Thanks

Stefan Bennett

Example follows

--Create the tables and insert the values
CREATE TABLE Sales_West (
Ordernum INT,
total money,
region char(5) check (region = 'West'),
primary key (Ordernum, region)
)
CREATE TABLE Sales_North (
Ordernum INT,
total money,
region char(5) check (region = 'North'),
primary key (Ordernum,region)
)
CREATE TABLE Sales_East (
Ordernum INT,
total money,
region char(5) check (region = 'East'),
primary key (Ordernum,region)
)
CREATE TABLE Sales_South (
Ordernum INT,
total money,
region char(5) check (region = 'South'),
primary key (Ordernum,region)
)
GO

INSERT Sales_West VALUES (16544, 2465, 'West')
INSERT Sales_West VALUES (32123, 4309, 'West')
INSERT Sales_North VALUES (16544, 3229, 'North')
INSERT Sales_North VALUES (26544, 4000, 'North')
INSERT Sales_East VALUES ( 22222, 43332, 'East')
INSERT Sales_East VALUES ( 77777, 10301, 'East')
INSERT Sales_South VALUES (23456, 4320, 'South')
INSERT Sales_South VALUES (16544, 9999, 'South')
GO

--create the view that combines all sales tables
CREATE VIEW Sales_National
AS
SELECT *
FROM Sales_West
UNION ALL
SELECT *
FROM Sales_North
UNION ALL
SELECT *
FROM Sales_East
UNION ALL
SELECT *
FROM Sales_South
GO

--Look at execution plan for this query
-- This correctly only accesses the South partition
SELECT *
FROM sales_national
WHERE region = 'south'

-- Look at execution plan for update
-- This accesses all partitions - Why?
update sales_national
set total = 100
where ordernum = 23456;