'Course, they could just be taking Alan Dean Foster as canon and assuming that there were OTHER Atmosphere Processors on LV-426. They may have colonies too which would be much less damaged than the one at the base of the explosion. They could easily say the opening is taking place there and not damage the storyline too much.
Which is nonsense, innit? But no-one is going to know until the developers explain their reasoning, so I figure it's open season for nonsense just now!