Because they had analysed the planetary atmosphere first and found it to be similar to Earth's conditions? As in Prometheus's case, a special edition with deleted scenes will make more sense out of this Covenant mess.
Watching and reading horror, hanging out at horror.com
Favorite Horror Movie
Halloween, Evil Dead, An American Werewolf in London
Signature
I command by the power of Christ and the Gospel for this evil spirit that is inside this man or woman to come forward now and to face the judgment of God.