Assigning “Read” access to a delegate assign automatically "Send On behalf" right

Hello,

In Outlook 2010, when I assign “Read” access to a delegate, the right "Send On behalf" is assigned automatically to that delegate in mailbox properties (Exchange 2010 console).

Is this a bug or a default behavior of Outlook and Exchange ? Is there a way to avoid giving the right "Send on Behalf "automatically when I assign “Read” access ?