Airplane seat belts and Passengers

Everytime I board an airplane, the captain of the aircraft always annouces the following (or a varation thereof):

(seat belt indicator lights turn on) This is the captain speaking. We are almost at the end of our flight and will be starting to decend in a couple of minutes. The weather is ....weather report.. Please put your seat belts on and keep them fastened untill the seat belts indicator light turn off.

The indicator lights only turn off after the plane has halted on the terminal and the gangway has been connected. However, the passengers never forget to take off their seat belts as soon as the plane lands and and starts to go slowly towards the terminal.

Why is it that they cannot understand the clear instructions given by the captain? (the instructions are usually given in languages spoken at the ports of takeoff and landing). A clear and draconian solution to this is a master switch which controls the opening of all the seat belts once they have been fastened just before landing. This way the passenger safety can be guaranteed with no mishaps in case there is a big bump while on the way to the terminal.

PS: Yes, I'm mean ;)