I agree with AllieKat. It's not just the acceptance problem with mag stripe cards, but EMV does have additional security benefits. If Walmart is allowing swipes of EMV cards, then the security benefits are negated. If someone clones the mag stripe on an EMV card, they'd be able to use it at Walmart now whereas they'd be forced to insert the chip, which they wouldn't be able to clone, if Walmart were enforcing the service code.
True, we have effective no liability in the US anyway, but dealing with a stolen card is still something we'd rather not have to do.