The sign was easy to understand. It referred to the sound of someone patting and tapping the watermelon before choosing it.
The supermarket thought it would destroy the fruit, so they put up a sign forbidding people from doing so.
Originally, this kind of thing was nothing objectionable. The watermelons belonged to the supermarket, and they had the right to set the rules in their business.
However, the supermarket put the sign in Chinese characters, which caused some Chinese people to complain. This was obvious racial discrimination and also an insult to the Chinese!
Maybe the people who tapped on the watermelons before were Chinese, but who said that all the people who did so were Chinese? Writing the sign in Chinese implied that Chinese people were ignorant, primitive foreigners.
