Well, since we didn't see the beginning or the end of that scene in Alien, it's totally open to what actually happened off screen.
With respect, I have to disagree. They're cocooned for something
and if it was merely to hold them (why?), Dallas wouldn't have been nearly so sickly or asking for death, etcetera. He would have simply been pulled out, like Newt was.
And it's damn obvious that a lot more than mere cocooning happened with Brett, wouldn't you say?