Where are you located? I am not aware that this is part of the standard residential code in the United States. It must be specific to your local area.

I'm actually Canadian. I heard this recently from a friend of the family that custom built a small bungalow. They wanted a walk-in shower and no tub due to a health issue, but had only one bathroom on their main floor. They had to put a tub in the basement bathroom. I was just wondering if there was some reason for it. I think it's sort of odd.

Even in most of the people living in the home are adults (and therefore probably take showers), most people will include at least one tub for future buyers who may have infants and want to bathe infants/toddlers in a tub. Wasn't aware that one was required by code though...