Ugh... I did look up the case law after reading your comment, and according to the FCC you are correct - their definition does include butts.

The Commission has consistently interpreted the term "sexual or excretory organs" in its own definition of indecency as including the buttocks, which, though not physiologically necessary to procreation or excretion, are widely associated with sexual arousal and closely associated by most people with excretory activities. Thus, the Commission has in many cases treated naked buttocks as coming within the scope of its indecency definition, even though it has not always concluded that particular depictions or descriptions were patently offensive and thus actionably indecent.

• Complaints Against Various Television and Licensees Concerning Their February 25, 2003 Broadcast of the Program "NYPD Blue"

My favorite part of the document is this.

Moreover, if we interpreted these terms in the narrow physiological sense advocated by ABC and the ABC Affiliates, the airwaves could be filled with naked buttocks and breasts during daytime and prime time hours because they would be outside the scope of indecency regulation (at least if no sexual or excretory activities were shown or discussed).

Won't somebody please think of the children?

But I could swear I've seen naked butts on broadcast TV since the rating system was implemented.