Trivia: Much of the ending was reshot, including a scene where the alien pilots within the spaceship were witnessed by Kate Lloyd dangling from some sort of tubes; the body of one was replaced post-production by an odd Tetris-like display.
Trivia: The showdown with the Dr. Sander Halvorson-thing was done as a disappointing rush job when the ending was hastily replaced - Kate Lloyd was meant to encounter the spaceship's pilots and a whole other confrontation was devised, along with Dr. Halvorson being dead.
Trivia: Early script ideas for the plot included having MacReady and Childs rescued but frostbitten and then the horrid truth about one (or both) revealed, or having MacReady's helicopter pilot brother come by the camp and discover the disaster there. Director Matthijs van Heijningen Jr. wanted to remind the audience of MacReady with the character of helicopter pilot Sam Carter (played by Joel Edgerton).
Answer: By disabling the snow vehicles, they are making sure that the creature can't use them to escape from the quarantine zone. Since the creature can assimilate anybody, it would probably know how to use one.