Are you absolutely sure that the school finds out they won't be attending due to not showing up to orientation? That's a very inefficient way of finding out when an accepted student isn't matriculating.
I think if the tuition isn't paid by a certain date before matriculation, then the school knows you're not attending, or they will contact you to see what's going on. The circumstances you are speaking of doesn't happen very often. I've heard students (very few) being accepted a couple of days before orientation, but not when orientation starts.
I think medical schools know this and have a "protection" in place for situations like these. I never notified of my withdrawals to schools, but some ended up contacting me after the due date asking me if I was really matriculating or not.
Yes, it might be inconsiderate for students who don't want to attend the school but don't withdraw later, but considering that the deposit is a massive $3000, I would hold on to that as long as possible, if I were placed in that situation.