They are contracted by WalMart to build tires the the very barest of government standards. That's why treads can vary, etc... They do meet safety requirements, but the ones (Viva, etc.) that you find only at Wally World are the bare minimum to keep cost down.
The other variable is the shop- I got a set of rear tires on my Thunderbird a few years back at Wally World so I could sell it, and it took them 5 times to get them balanced right, even with a Hunter balancing machine. That worries me as much as the tires, since they're mounting them. That isn't to say that the techs at your local Wally World aren't completely competent, that was just my experience.
I guess at this point, I know that cost enters into the equation for a lot of people as money is tough, but being that the tires are the only thing between my family and the highway, I buy standard issue product from a reputable tire shop. A lot of people curse off the Chinese tires as being uber dangerous, but the Wally World generics like Viva are meeting the same standards. Those aren't good enough for me and my family. That's just my .02 though.
Remember- when tire companies are competing against each other, you get better product- just like cars. Ford got better because of competition. But when you're awarded a contract based on price and bare government standards, what is your incentive to innovate your tire? Pay more, you may get way above the gov't standard, since they're trying to get people away from Michelin, Cooper, BF, etc. at the tire shop- it's about features, and innovation. At WalMart, it's all about the money.
There is a great story about how Snapper mowers stopped selling to Walmart because the CEO realized they would have to destroy the quality to compete
That will be happening more and more I think. Customer loyalty is important now. People have less money to spend, and they're starting to realize that buying 2 at WalMart isn't as good of a deal as buying a better quality item the first time.