SP Permisions

lets look in this simply SP:

---------------------------------

CREATE procedure test
as
Declare @cmd nvarchar(1000)

SET @cmd=N'Select * from Users'

exec sp_executesql @cmd

---------------------------------

User has rights only for executing SP in my DB. So my question is: why he can not execute this SP??? (error with permision on Users table). It is strange and now I have to add SELECT permision on USERS table for this user. This SP is running without problems:

CREATE procedure test1
as
Select * from Users

So why 'test' isn't running???

Fly