Imagine a hypothetical situation where there are a finite number of sale items. This is advertised to the public, causing masses flock to the store. However, upon arrival, none are available, as the stores' employees have "taken advantage of it".
Would this not be akin to false advertising? As from the perspective of the consumer, no stock was actually available to them.
The idea of employee perks should not interfere or diminish consumer experience. Any competent manager would not allow staff to clear a product that is in high demand, as it's just bad business practice.