Same thing. Dimetapp and Sudafed both contain pseudoephedrine. They are both still available without a prescription, but they're now behind the counter and available on a limited basis.
The pseudoephedrine contained in these products and others were being used illegally to manufacture methamphetamine. Rather than banning these products or making them available by prescription only, Congress chose instead to merely limit the sale.
As a result, meth production has dropped significantly. This means fewer labs blowing up, fewer children poisoned by the process, and fewer expensive environmental cleanups.
"Actually I do get by pretty good with the Walmart brand of Afrin."
You're lucky. They only thing that worked for me was Tavist-D, and the FDA pulled it.