yeshivahguy: The Minhag HaPashut until the 1970s was that peanuts were not Kitniyos. There was a bit of a political battle and, to quote a major Yeshivish Posek, “The Kashrus organizations caved to the Machmirim.” There are still many who in theory would hold that peanuts aren’t Kitniyos, they just won’t eat them because they don’t have a Hashgacha. Thus, some Rabbonim are allowing Ashkenazim to but food with an OUKitniyos/Batatz L’ochlei Kitniyos if peanuts as the only problematic ingredient.
As an aside, R’ Moshe’s T’shuva is based on an incorrect historical assumption. R’ Moshe says explicitly that the reason that potatoes are Muttar and corn is Kitniyos is that corn is an old world grain while potatoes are a new world grain. This isn’t true. Corn wasn’t introduced until the very late 15th century, when they found it in American. There are several ways to explain the fact that corn is Kitniyos and still hold of R’ Moshe’s general rule that nothing new can be added to the Kitniyos G’zeiros. However, the fact is simple that no one in their right minds would think that corn isn’t Kitniyos.