I too fail to see how it is Wal Mart's fault or problem that this guy did not know what ammunition his gun used. It is impossible for even the most knowledgable person to be able to know exactly what cartridge any gun someone might have shoots, much less if they never even see the thing. Consider for a moment how frustrated the customer might have been when the .357 Sig ammunition that the "properly trained" Wal Mart employee sold him (.357 Sig is the semi-auto cartridge, remember) didn't fit in the magazine, much less chamber, in his Desert Eagle or Coonan (both of which are chambered for .357 Magnum).
Quite frankly, I feel the blame lays on the guy who didn't know what ammo to shoot in his gun. The guy was stupid plain and simple, which is quite different from ignorant. All of us are ignorant about something at some point. I've come across many firearms which I did not know the chambering for, thus I was ignorant. I was not stupid, however, because rather than take a guess or expect the Wal Mart gun counter employee to know, I researched the gun and learned what the proper cartridge was and therin lies the difference: a stupid person is someone who is unwilling to learn.
In this particular case, the guy with the Glock was unwilling to invest even minor effort into finding out what the proper ammunition for his gun was. Finding out such information, particularly on a fairly new and currently produced gun like a Glock, is quite easy and requires only minimal time and effort. I suspect that simply reading the owner's manual would have cleared up any misconceptions that the man had. If the gun did not come with an owner's manual, they are usually quite easy to obtain by either requesting one from the manufacturer or downloading one from the manufacturer's website. Even if an owner's manual is unclear or unavailable, the information still isn't that hard to obtain. A simple telephone call to the manufacturer will allow you to talk to someone who can reasonably be expected to be knowledgeable about your particular firearm that will, in all likelihood, clear up any misconceptions that you may have. Even if you can't talk to the manufacturer about your gun (out of business, discontinued model), the information age dictates that the knowledge you seek is probably no more than a few keystrokes and mouse clicks away. In most cases, simply registering on one of the many firearms forums such as this one will put you in touch with several knowledgable people who are more than happy to answer any questions you may have. The stuipid man in question, however, couldn't be bothered to do any of this.