The crew in Alien are essentially normal, everyday people finding themselves in a situation that is way out of their depth. Ash is basically the scientific and medical expert. The rest of the crew look to and rely on him on these issues. Obviously he has alterior motives so he deliberately engineers the situation in service of his motives.
Ripley is a very capable officer, she makes a stand when it comes to following quarantine procedures when they return to the ship. Next day when the facehugger drops off she conveniently forgets all these objections and just welcomes him back without so much as a scan.
It's patently ridiculous but you gloss over it because movies.