In the Chinese Wikipedia article on sky burial, there's a section saying that sky burial is illegal in Japan:
Dumped suspect [My assumption for translation: Illegality of abandoning bodies]
In some places, burial behavior may violate abandoned corpses offense.
Penal Code section 190 (dead body damage Huai crime)
Did Japanese Buddhists once use sky burial for their dead? Or is the ban just a broader ban that just happens to include sky burial?