Maybe I'm misremembering but all I recall was that that Mother did some preliminary reconnaissance on the planet and it was deemed earthlike and habitable but that doesn't preclude the possibility that they could have expose themselves to indigenous pathogens.
I'm fine with that suspension of belief. They analysis the planet, it was determined safe, etc. They don't want to go back to sleep for 7 more years, Omega whatever was still just a guess if it was habitable, and then this showed up. It was like a godsend to them.
This was very Alien. The discussion on protocol with Kane's containment, checking out the site, etc. Human decisions, a bit dumb ones looking back. But Kane had a bugger on his face and then they just let him walk around after it magically drops off?