One of the most important sins of Morgoth and Sauron was their desire to control the actions other beings, surely the curtailing of orcs' free will was merely another manifestation of the sin of domination.
Sin has a very specific meaning.
Orcs have free will when not under the control of Melkor or Sauron. Free is a relative word. Otherwise you could say the same for humans for Melkor also corrupted Men. You were already there when a few month ago we talked a bit about that. Melkor did things that inherently diminished Men.