I'd be more inclined to suspect that the sheer square footage of their stores makes it cost-prohibitive to outfit the space with the sensor infrastructure (comparing to Lululemon, who has much smaller stores). The tags themselves are fractions of a penny (I'm seeing $0.002 on alibaba) and are just applied as stickers, so I don't think that's too much of an issue -- not to mention Walmart probably has the buying power to insist their suppliers pre-tag all items.
