I believe that this dates back from thousands years ago. Food was scarce in China and Chinese people found out many ways to keep food.
The vegetable are kept in vinegar and together with spicies.
The salesman was handing out for free preserved vegetable while shouting "eat, eat, eat" "eat, eat, eat"... He knows that if customers eat, they will also buy.
While there are many preserved vegetable in China, there are no pickles except at Mac Donald...