I suspect it was cheaper and easier than trying to put a lockwasher or using a nylon locknut.
Here's the method I found easiest, after trying to grind them off and cut then off.
1. Mark the position of the frame and crossmembers.
2. Cut through the floor with a circular saw just outside the frame members.
3. Use a Sawzall to slice horizontally through the remaining plywood and screws.
You can see it here: http://www.airforums.com/forum...9&postcount=79