Stores could restrict employee purchases if they want, but the discount can't be stopped.
When I worked at Wal-Mart (almost 10 years ago now) the manager wouldn't let us buy hot items that had low availability. I don't think they could block it in the system, but I figured they could track purchases with the discount card and didn't want to break policy. I tow the company line where ever I work any ways.
Some of the Walmart employees, such as managers and pharmacists, make some of the top salaries in their field, at least in my area. My girlfriend works part time as a Walmart pharmacist (making as good wages as her full-time pharmacy job), so if her discount applies, we will be getting an iPad 2 from Walmart.
I remember talking to a vzw rep a while back when they first started selling the iPad 1. I asked if they got a decent discount - she told me, that there was no employee discount for the iPad 1. I can only presume the same for the 2.