Wal Mart is doing some pretty cool stuff with technology. Maybe not on the level of Amazon, which might end up biting them in the long run, but Wal Mart definitely knows their way around a database.
Some people at wal mart know their way around a database (database being euphemism for a broader "tech culture"), they are not well represented in management and wal mart is trying to change that (with jet acquisition). but too little too late.
Imagine a wife/girlfriend who is PISSED because you won't get into a heated argument, you just stay all cool and level headed, hedging statements and whatnot.
Wal-Mart already has a system in place to ship tens of thousands of different SKUs to within 10 miles of a huge number of consumers, at a low retail price. Amazon's system can handle millions of SKUs, but only to (maybe) somewhere within the same country as a consumer, relying on shipping businesses to move the product the remaining 0-2000 miles to the consumer. And in many cases, the cost of shipping (and warehousing) is already baked into the sale price, so that "free shipping" is actually "don't pay more for shipping than you're already implicitly paying."
When Wal-Mart says "free shipping", they are likely eating the cost for that last 10-50 miles, just to undermine Amazon, and may later invest in a delivery truck or two at select store locations to eventually cut out UPS, FedEx, and USPS. Or they might just be saving money already, because you no longer need to be served by cashiers and store security as you shop in-store.