My assumption has always been: people know pseudoephedrine works, so that's what they want to buy, but you can no longer sell pseudoephedrine over the counter. However, you can offer phenylephrine, which still lets you reasonably name the drug "Sudafed PE" or whatever. Customer browses the shelves, sees "PE" in the drug name, this triggers a vague impression that it contains pseudoephedrine and thus will actually work well.