Well, I think they only specify the setting when there's some concern about burning something on the bottom as it's coming to a boil. I wouldn't think it would matter with peanuts. It would matter a great deal with milk. Or maybe when there was just something barely covered with water that would be in contact with the bottom of the pan. So the instructions are to use "medium" to bring it to a boil. And you adjust from there to keep it boiling, if that's the plan. "Medium" is hard enough to define. No one can say which setting on every range would hold it to a low boil. As you see in the other thread about a weak range burner, some have trouble holding a boil at all. On my gas range, I need a heat defuser to hold even the "simmer" burner down to a simmer.
I would not, however, assume that just because a recipe didn't say to use medium to bring a pan of milk to a boil I could blast away on high heat. It may not occur to the author that someone might not realize they would be scorching the milk on high before it all boiled.