I don't quite buy the explanation given in this post, although it was an interesting read. I think Bombadil is beyond good and evil, he just is. From http://scifi.stackexchange.com/questions/1586/who-or-what-wa...:
"And even in a mythical Age there must be some enigmas, as there always are. Tom Bombadil is one (intentionally)."
The Letters of J.R.R. Tolkien, No 144, dated 1954