The verdict that water (bath) is necessary when there is emission given by the people (in the early days of Islam) was due to the concession granted by the Messenger of Allah in the beginning of Islam. He then commanded to take a bath (in such a case). Abu Dawud said : By Abu Ghassan is meant Muhammad b. mutarrif.
أَنَّ الْمَاءَ مِنَ الْمَاءِ كَانَتْ رُخْصَةً رَخَّصَهَا رَسُولُ اللَّهِ ﷺ فِي بَدْءِ الإِسْلاَمِ ثُمَّ أَمَرَ بِالاِغْتِسَالِ بَعْدُ