All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
A Muslim should be confident that in Islam, nothing is legislated without a benefit, whether this benefit can be realized by people or not. A Muslim must submit in full obedience whether or not he realizes this benefit.
Washing the dead person is, first, an act of worship and, second, an a procedure aiming at cleanliness so that the Muslim leaves this world in the best condition of cleanliness, as Ibn Qudaamah and other scholars said. That is why the Prophet recommended that the washing of the dead should be done repeatedly according to the need. He recommended the use of sidr (lotus-tree leaves) and camphor. Furthermore, washing the dead body rids it of decay, insects and quick decay. Washing the dead is, no doubt, an Islamic ritual. For more details about washing the dead, see Fatwa 84697.
As for disbelievers, we don't know what they do with their dead, and we don't care about that as we are sure that their dead, if they die in a state of disbelief, well be immortally in Hellfire. So, it doesn't matter what they do with them after their death.
Allaah Knows best.