The homemade concoction that becomes Meth uses over the counter cold meds, allergy meds, etc. The hillbillies who make that stuff were going into Walmart, etc and buying tons of the otc meds. Once law enforcement started noticing the pattern, they decided to pull it off the shelves, or quantities of it so you had to show ID. It was an easy way to try to track who was buying large amounts in short periods of time. At least it was a cheap way to track, with mimimal inconvenience to the regular consumer - usually government involvement turns into a huge time and money waste.
I didn’t know that, thanks