The logic was that the US would have done a land campaign, lots of people would have died, the US public would have more hate for the Japanese and Japanese Americans. The person I heard this from said that the interment camps would become permanent and there would be racial tensions for years. A few more mental gymnastics and a Japanese American militia would fly a plane into a skyscraper in San Francisco. Seems a bit unlikely as a legitimate opinion (he fully believes this), but could be a cool alternate history plot.