Unless I'm mistaken about the meaning of nawafil in Shiite context, I'm a bit confused about why Sunnah prayers for Sunnis are nawafil for Shiites.
From this site:
Misconception #4: Shias do not perform sunnah prayers. Sunnah prayers are non obligatory prayers performed by Prophet Muhammad (S.A.W.).
Response: Shia do perform non-obligatory prayers, 36 cycles per day in total, but call it nawafil and not sunnah.
Sunnis have both nawafil and sunnah prayers. So, what is the difference? Why are Sunni sunnah prayers nawafil for Shiites?