It's pretty simple. If a home owner buys an A/C and it has a 20 amp 125v 2p 3w male cord and they try and plug it into a 15 amp receptacle, they see it doesn't fit. Next logical step is to call an electrican who tells them you need a 20 amp circuit for that apliance and explains that you can't just change the receptacle because the circuit won't hold.
