That's because pseudoephedrine is a convenient starting material for basement lab synthesis of methamphetamine.
I'm aware of that. However, the WalMart policy - as programmed into their cash registers - is to disallow a purchase of more than 3 items containing pseudoephedrine. Doesn't matter how small the items are or whether the product could practically be synthesized into meth. No, instead they disallow anything over three - the cash registers refuse the transaction. I spoke to the manager of the WalMart today and he said that they've now programmed the registers to still refuse the transaction and to also spit out a piece of paper explaining the policy. He said that this is an agreement that WalMart has with the FedGov.