This situation I described in the OP is either a poor landing page decision by the merchant or some attempt by the merchant to "take advantage of" publishers. I don't know which and if there is another possibility, I don't know what it is.
It's tempting to email the merchant and suggest clicks on an ad for green widgets should land on a page where green widgets are sold, on the possibility the merchant is just ignorant.
But I guess Google doesn't want such contacts between publishers and merchants. Maybe they should consider some method of forwarding suggestions from publishers to merchants.