How do I sync only "checked songs" to my iPhone?

I have my iPhone settings in iTunes already set to sync "only checked songs" in the Summary section, and it used to do that. But with the new iOS6, and some recent purchased music downloads, the options in the Music section override that. I only have the choice of synching the entire music library (which fills up my iPhone), or synching by artist, album, genre, etc. I really don't want the hassle of creating playlists. Why is this overriding my "only checked songs" choice, and not doing that anymore?