I use to do a little skydiving. We were trained and required to use seat belts until we were at least 1500' above ground level. Any lower and we would be unable to evacuate everybody and get our chutes open in time. It takes somewhere from 200' to 700' of freefall for a chute to properly deploy, not to mention the extra time it would take to get evetybody out the door.. Also, not all planes have the doors removed. Most jump planes have doors that are modified for skydiving purposes.