Question About EXECUTE AS USER

Can anyone tell me why I'll receive a error via following steps? Thanks in advance!






1. Create a database €œTESTDB€? and a table €œTable1€?

2. Create a login €œTestLogin€?, which is db_owner roles of both msdb and TESTDB


3. Create a DML trigger for Table1 by following script

CREATE TRIGGER TRG1
ON Table1
FOR INSERT, UPDATE, DELETE
AS
SELECT * FROM msdb..sysjobs
GO


4. Open SSMS, login as TestLogin, execute following statement

SELECT * FROM msdb..sysjobs

It will succeed to select data in msdb..sysjobs

5. Open another SSMS, login as sa, execute following statement

use TEST
execute as user='TestLogin'
select * From msdb..sysjobs

I receive an error about permission to select data from sysjobs, why?