The Rambam opens Hil Ta'anis by telling us:
יש שם ימים שכל ישראל מתענים בהם
He then lists all the fast days and tells us what happened on each of them. In halacha 5 he writes:
וארבעת ימי הצומות האלו--הרי הן מפורשין בקבלה
What's important is what the Rambam does NOT say -- he never writes that there is a chiyuv m'divrei kabbalah to fast on any of these days, even on 9 Av! The fasts are described, "mefurashim," in divrei kabbalah, i.e. Nach, but the chiyuv to fast is simply based on minhag yisrael and no more than that.
Contrast that with the language of the Tur, "Hakol chayavim l'hisanos m'DIVREI KABBALAH..." True, the Tur quotes the gemara that when there is no shmad the chiyuv to fast depends on the ratzon of Bnei Yisrael, but it seems that according to the Tur the idea of ratzon is not a mechayeiv (which is how the Rambam must have learned the gemara), but is just an acceptance of the chiyuv that already exists m'divrei kabbalah.