I think it's because they previously thought that eating peanuts during pregnancy meant that he baby was more likely to have a peanut allergy. They since found no evidence, so in the UK at leas, we are told we can eat peanuts as long as there is no history of peanut allergy in the family.
Ok... just wondering because there was nothing about peanut butter/nuts in any of the guidelines I was given. I'm in the US.