There's only one reason phenylephrine is used as a substitute in the OTC decongestant products: the general public can't pronounce or spell the word "pseudoephedrine" and they're intentionally being duped.
They walk into the grocery store, convenience store, or pharmacy after a sleepless night of breathing through their mouth, see the red package that says SUDAFED and a vaguely familiar-looking active ingredient, grab it, and plop their money on the counter.
PsEudoEPHedRINE
PhEnylEPHRINE
This is OTC medical fraud.