Row Level Security in MS SQL

Dear Josh,

You just got back from the excellent Kimball DW/BI Lifecycle in Depth in NYC, and one of the concepts briefly mentioned was row level security.  You made a note to look into this and came up with this demo script:

/* Companies table -- I should only be able to see the customers in my company. */
   DECLARE @Companies TABLE (
       CompanyId INT IDENTITY NOT NULL,
       CompanyName VARCHAR(50) NOT NULL
   );

   INSERT INTO @Companies
       (CompanyName)
   VALUES
       ('My Company'),
       ('Not My Company')
/* Customers table */
   DECLARE @Customers TABLE (
       CustomerId INT IDENTITY NOT NULL,
       CustomerFirstName VARCHAR(50) NOT NULL,
       CustomerLastName VARCHAR(50) NOT NULL,
       CustomerDateOfBirth DATETIME NOT NULL,
       CompanyId INT NULL
   );

   INSERT INTO @Customers
       (CustomerFirstName, CustomerLastName, CustomerDateOfBirth, CompanyId)
   VALUES
       ('John', 'Smith', '8-1-1988', 1),
       ('Abe', 'Jones', '6-15-1951', 2),
       ('Mike', 'Bullard', '11-8-1960', 1),
       ('Joan', 'Smith', '1-11-1987', 2),
       ('Albert', 'Simpson', '3-3-1980', 1),
       ('Frank', 'Taylor', '9-12-1962', 2),
       ('Mo', 'Robinson', '8-12-1953', 1),
       ('Tina', 'Sommer', '2-14-1978', 2)
/* SalesPeople table */
   DECLARE @SalesPeople TABLE (
       SalesPersonId INT IDENTITY NOT NULL,
       SalesPerson VARCHAR(50) NOT NULL,
       CompanyId INT NULL
   );

   INSERT INTO @SalesPeople
       (SalesPerson, CompanyId)
   VALUES
       (SUSER_SNAME(), 1),
       ('Not Me', 2)
/* My Company's customers */
   SELECT
       CU.CustomerId,
       CU.CustomerFirstName,
       CU.CustomerLastName,
       CU.CustomerDateOfBirth,
       CO.CompanyName,
       SP.SalesPerson AS SalesPerson
   FROM
       @Customers CU
       INNER JOIN
       @Companies CO ON CU.CompanyId = CO.CompanyId
       INNER JOIN
       @SalesPeople SP ON CO.CompanyId = Sp.CompanyId
   WHERE
       SP.SalesPerson = SUSER_SNAME()
/* Not My Company's customers */
   SELECT
       CU.CustomerId,
       CU.CustomerFirstName,
       CU.CustomerLastName,
       CU.CustomerDateOfBirth,
       CO.CompanyName,
       SP.SalesPerson AS SalesPerson
   FROM
       @Customers CU
       LEFT JOIN
       @Companies CO ON CU.CompanyId = CO.CompanyId
       LEFT JOIN
       @SalesPeople SP ON CO.CompanyId = Sp.CompanyId
   WHERE
       SP.SalesPerson <> SUSER_SNAME() OR
       SP.SalesPerson IS NULL