Scat actually isn't listed under 'Prohibited Content' in the ToS. The person who edited the Uploading Guidelines may not have realized it. (Or maybe the ToS changed since then? That edit was 3 years ago.)
Posted under General
@soryuu9: Actually it is listed. Under Prohibited Content-Grotesque, feces is on the list of what falls in that category.
Like a lot of Danbooru TOS, it's meant to discourage shitposting (forgive the pun).
Furry is also "prohibited" but is approved at the discretion of the moderators.