For an employer there is a real cost associated with finding and hiring a new employee. The worry is that someone who is over-qualified is going to continue to be looking for higher-paid/higher-skilled work that is a better fit for them. That employee will also be generally less concerned about their future career path at the job they are over-qualified for, which can lead to reduced quality of work. In a customer-facing role at a retailer this can come through as a bad or passe attitude, which won't win over your customers.
No one's arguing (well, not much) that the employers don't have a decent reason for this. The problem here is that the incentives of employees and their employers are becoming more and more at odds with eachother.
