Walmart phrased its recent open carry ban as a request, the same as Target and other retailers did after the long gun open carry events in Texas. Such a "request" in many of those businesses that in 2014 were said to have banned open carry have not been enforced in Nevada. I openly carry in Target all the time, from 2014 onward. Even Walmart's door signs say "kindly refrain," a far-cry from the "do not openly carry or you'll be arrested" 30-07 signs in Texas.
So why does your average Walmart flunky care all of a sudden?
1. Management pressure. I'm just speculating here, but back-channel communications from Bentonville or at the local level may have been to instruct staff to tell open carriers to leave, cover up, or trespass them. So the staff takes it to heart, fearing for their job, and carries out the order, lest their manager find out they "pretended not to notice."
2. Fear/hatred of guns. A lot of Walmart employees in urban are from poor economic backgrounds where they may have had no positive access to firearms. They may think carrying guns is stupid, open carry is bad, believe the news media exclusively, etc. Just your standard person who is ignorant of facts.
3. Afraid of something bad happening and not stopping it. Guilt is a powerful motivator. This is reasonable, yet the average person should be able to tell the difference between an open carrier with a holstered handgun, a weirdo toting an AR-15, or this guy.
4. Wanna be tough guys/exercising power. No offense to anyone who works at Walmart and is reading this, but a standard Walmart position puts your pretty low on the social totem pole of jobs. You have little power or discretion in the world and you know it.
Wanna see this in action? Try ignoring the greeter who wants to paw through your shopping bags on the way out while checking things off on your receipt. Some of them will get really pissed off because they know they can't make you stop or lay their hands on you. The actual selection of who gets checked can even be discriminatory and petty.
Most people like exercising power in some way. For a minimum wage person at Walmart, being allowed to tell people what they can and can't do is validating. As one local gun owner put it on a local forum:
So, I was just in my local Walmart and asked the sporting goods guy if Walmart's opinion on OC has softened at all? He actually said not at all and that he has told people to get their gun out of the store or he would take it and slap them with it!
People want to be tough and exercise their powers. Often people make hyperbolic statements to emphasize their opinions or to lend themselves credibility (they think).
5. They aren't smart enough to question orders. Let's face it, most Walmart employees aren't that bright. If you're a Walmart employee reading this, don't get offended, you're obviously smarter than most and have your reasons for working there. A job is job, but then again you know who I'm talking about. There are some people who will jump off a bridge simply because you told them to. So if told, or it is implied they should tell people with guns to leave the store, they'll probably walk up to the killer who there to shoot up the store and tell them to leave, only to be the first to die.
Less dramatically, there are people who will follow 1. (above) and tell people to leave. They just aren't bright enough to look the other way and exercise discretion not to care.