Answer and Explanation:
The answer is explained in the very next paragraph.
The manager does that because "That poster was delivered to our greengrocer from the enterprise headquarters along with the onions and carrots" (Excerpt from the passage)
Basically what they want to convey is that, the people they never understand the meaning of most of the slogans. He did it because it had be done for years and everyone does it.
That's how things work. If he refuses, he could get into trouble.