If they actually do that, it'd be considered unethical. While specific credit isn't always given to the originator of an idea, superiors shouldn't pretend they actually had the idea. In many organizations, explicitly passing off an idea as your own would be a pretty serious offense.
There's a big difference between passing someone's idea off as your own, and allowing someone to pass an idea off as their own.
I guess I'm considered "unethical" around these parts, but I stand firmly in the court that the buyer may have ethical issues, but certainly not the seller.