I don't think most walmart employees are stupid or retarded. They just go by the rules set for them and do not wish to get in trouble or lose their jobs. I worked for walmart for seven years and I know a lot about guns. But walmart has their policies and employees follow them as did I.
I never buy ammo from walmart any way except for when I worked there for the 10% discount. When it comes to firearms and ammo I go to a place that actually knows about that kind of thing. The employees are not trained about firearms and the ammo. It is not their job to know federal regs. All they need to know is walmart and local store policy. And then there is the simple fact that walmart is a corporation and if they choose not to sell the 9 mm to those under 21 it is their right. They are not violating anything by doing this. We may not agree with it but that is life. They don't have to sell any ammo to any one under 21 if they don't feel like it. I was even told once that if I did not feel comfortable selling ammo or a firearm to someone I did not have too.
Also the 50 cal ammo is a cute touch but in the seven years I worked for them and all the years after I have never seen 50 cal ammo in walmart. Seems a bit overboard to make a point. stopping at the shotgun shell would have better. And busting on the employees just trying to make a living is sad. We are all just trying to get by and keep our jobs in this economy. I hardly find that stupid or retarded. You don't have to be a rocket scientist to work in retail. In fact I admire most of them for being able to put up with pain in the arse customers who think they know walmart policies or think they should be treated like royalty.
P.S. Managers at walmarts are allowed to adjust many of the policies within reason or for what they feel will keep customers and employees safe. If a manager does not want weapons in their store complaining to corporate will not matter. They will let the manager know of your complaints but it will still be up to the manager. Same with any other issue.