the logic behind such practice IS pretty simple.
they (& other companies) are producing games/vns primarly for the japanese market, means aiming at japanese customers (or others affiliated with their culture) in general, which requires adjusting to their natural habits & also strictly considering their preferences (for maximizing profit).
And thats also the main point why many publishers refuse working together with western ones, as they fear that that kind of content, once made accessable to the west, might affect their countries image in a very negative way (Like the RapeLay skandal did). As a direct result of this people in japan might refrain from puchasing their products, out of fear beeing stigmated by citizens of the western hemisphere - ergo leading to a massive money loss for japanese game developers.
we can say what we want, but from a conservative point of view the content presented in 90% of Vn´s is considered highly desturbing porn, often paired with violence.
Blame it on your goverment, curse your mother for teaching you morals & ethics, despise everything, but it´ll stay the same, as long as the western world wont become more openminded & less prudish regarding sexual content.