It's a classic heads-I-win-tails-you-lose tactic. You put a rule in place that overtly punishes people who do something unethical for the benefit of the company

It's a classic heads-I-win-tails-you-lose tactic. You put a rule in place that overtly punishes people who do something unethical for the benefit of the company (in this case, pestering their colleagues while they are on vacation) while at the same time you put your employees in a situation where they must break the rule to keep their jobs. That way management gets the benefit of employees engaging in unethical behavior in secret, while on paper everything looks clean and management is not responsible.