Q
Why do I have to give permissions on the underlying database objects when I'm using dynamic SQL statements?

A
These permissions are required for security reasons. Consider the following simple stored procedure:

USE pubs
GO

CREATE PROCEDURE GeneralSelectTableName SYSNAME
AS
EXEC ('SELECT * FROM ' + TableName)
GO

You probably expect that your stored procedure will make a call that looks something like:

USE pubs
EXEC GeneralSelect 'authors'

However, consider the consequences of someone passing to your stored procedure the following:

USE pubs
EXEC GeneralSelect 'authors DROP TABLE authors'

If you, the creator of the stored procedure, were a member of the db_owner role in the pubs database and your users needed only the EXECUTE privilege on the stored procedure, then this command would drop the authors table. SQL Server protects against such unauthorized actions by requiring that users possess the appropriate permissions on the database objects referenced within dynamic SQL statements.