"According to the Rambam in Sefer ha'Mitzvos (Shoresh Rishon; Lo Sa'aseh #312; Mitzvas Aseh #174), the prohibition of "Lo Sasur" -- 'Do not turn away from what they (the Chachamim) instruct you' (Devarim 17:11) -- obligates a person to keep the Mitzvos d'Rabanan."
My question is, why do you interpret 'they' to mean 'the Chachamim'?
Baruch Elasaf, Garland, Texas
The context of the Pasuk lends support to the Rambam's position. The Pasuk states that when there is a question of Halachah "u'Vasa El ha'Kohanim ha'Levi'im v'El ha'Shofet Asher Yihyeh ba'Yamim ha'Heim v'Darashta vihi'Gidu Lecha Eis Devar ha'Mishpat....Lo Sasur" -- "And you will come to the Kohanim the Leviim and to the judge who will be in those days, and you will search out and they will tell you the word of the Law...You should not deviate" (Devarim 17:9-11). This implies that anyone who does not listen to the Kohanim, Levi'im, or judge who tells him the Halachah transgresses Lo Sasur.
There is an argument to be made that this only applies to the Beis Din ha'Gadol in Yerushalayim, as the Pasuk says "Min ha'Makon ha'Hu" -- "from that place" (ibid. 17:10, see Minchas Chinuch Mitzva 495, which would include all Mitzvos and Takanos which they oversaw). However, not only does the context of the Pasuk seem to support the approach that this applies to any Mitzvos that are established by worthy Rabbanim, but additionally there is the teaching from "Asher Yihyeh ba'Yamim ha'Heim" -- "who will be in those days" that "Yiftach b'Doro k'Shmuel b'Doro" -- "Yiftach in his times should be considered as Shmuel (ha'Navi) in his times" (see Rosh Hashanah 25b). This means that we understand "Shofet" to include all Halachah that will be established by the Rabanan (Rashi quotes this Derashah on the Pasuk). This could be why the Rambam understands this as referring to all Mitzvos d'Rabanan.
All the best,