This is dealt with in the script.
Ash is the science officer and the although the other crew members disagree with his decisions regarding Kane, none of them actively rebel against his right to make those decision.
DALLAS : You're the science officer. It's your decision.
and
RIPLEY : How could you leave that kind of decision to him?
DALLAS : I just run the ship. Anything that has to do with science division, Ash has the final word.