In a Japanese home, putting soap or body wash directly into the bathtub is generally considered a clear no, especially when family members use the same bathwater one after another.
The basic reason is simple. In Japan, you wash your body and hair outside the tub, then get into the bath to soak. The water in the tub is not meant for washing. It is meant to stay clean enough for the next person to use.
If you add soap to the bathwater, the next person ends up soaking in soapy water. That can feel unhygienic, but it is also simply unpleasant. The bath is supposed to be a place to relax at the end of the day, not a tub full of leftover soap.
That idea matters because the bathtub is not just for getting warm. For many people, soaking is part of unwinding after a long day. If several family members are using the same water, keeping it pleasant for the next person becomes part of the way the bath is used.
Bath additives are different. Products made specifically for bathwater are commonly used. Even then, if a family has never used a certain bath additive before, it is natural to mention it first. Once everyone already knows that bath additives are used from time to time, people usually do not need to ask permission every single time. But if the bath suddenly has a strong scent or noticeable color, someone may reasonably think, “You could have told me.”
There can also be practical reasons to check the label. Some households reuse bathwater for laundry, so they may want to know whether a particular product is suitable for that. Bath oils should also be checked individually rather than assumed to be fine for every bathtub.
If you live alone and no one else is going to use the water, then what you put in your own bath is largely your own choice. That is the important distinction. Soap is not avoided because soap itself is somehow unacceptable. It is avoided because the bathwater is often shared, and everyone wants to be able to soak in clean, comfortable water.