I know that UT Austin did that last year - they had 30 places and sent out 90 admits. Since you've already received an offer from them, when you decide to reject it won't have much impact, unless they've changed for this year, because they won't be sending out another offer. As for the schools you're waiting for, I'm not sure which system they use, but I believe most top CS schools (especially the ones with larger departments that don't care if they yield the exact number of places) use this same mechanism.