I remember asking the same thing after watching Aliens when it was released. I assumed that the Nostromo by default transmitted information, its last position, etc., to be picked by the nearest ship or outpost. It's possible that the company did not receive the transmission; otherwise, they would have sent crews that specialize in investigating alien life forms to LV-426 while it was being terraformed. In which case, they found out about the alien first from Ripley.
I also assumed that the special orders to investigate life forms (or any resource) and to use androids like Ashe for a variety of purposes are all part of general policies meant to protect the interests of the company and to maximize profits.
Finally, about the marines' reference to bug-hunts, company officials mentioned that they had not encountered a creature described by Ripley in more than 300 surveyed worlds, but they could have discovered other life forms.