Online checkin seat number changed by airline without notice.

No matter what seat you choose when you check in online they can just move you when they feel like moving you. Why do you give me the option of choosing a seat when uou are just going to changed it. Think this is a bit pathetic espesially when you want to board and you are just informed that your seat has been moved, with the explanation of "this seat was paid for." Yes i also purchased a ticket from your airline.