For pilots to have BMI of 28 in the US and western countries, they have to be quite fit and in shape
Maybe, just maybe, that's what the Japanese are looking for when they carry out the medicals; quite fit and in shape as opposed to unfit fatties. Seeing the size of some of the US crews (and other countries of course) I actually side with the Japanese and don't think it is unreasonable for them to expect pilots to be the correct weight and not the size of a hippo.
As TD says, it doesn't matter what your opinion of BMI is, you either conform to their standards and how they want to play the game or you don't join.
Edited to add: As far as I know KAL take the same approach and the numbers are the same. The Chinese are less forgiving.