The employee discount wasn't the only reason from the article. There are a few cited. However, one of the few perks of working a retail job is that the workers tend to get some sort of discount on the products. Its a small token of appreciation, and yet has a two fold advantage:

makes your employees happier

encourages them to buy things at the store and use them, so they can in turn sell them and be more knowledgeable.

Taking away the employee discount may seem to be mostly innocuous, but it can really crush morale and accerlates employees skipping to the next job.