There is a reason.
The conveyer belt usually has a sensor so that it stops moving when something reaches the end.
( tangledupinblue hairy badge with moving eyes, Fri 15 Mar 2013, 11:50, archived)

SO YOU KNOW AND YET STILL FLOUT SOCIAL CONVENTION?
I hope someone snaps and punches you in the eye next time.
( The Baroness The Harlot Pimpernel, Fri 15 Mar 2013, 12:00, archived)

You only do it because your fear of your shopping getting muddled with someone else's runs in both directions.

( tangledupinblue hairy badge with moving eyes, Fri 15 Mar 2013, 12:14, archived)