did u tell them that your machine is still under warranty? I believe this is considered manufacture defect because the screw was not tightened properly, thus causing it to drop/fall off at casual usage. Reason with the technician, and tell them you will bring up to consumer court should they insist to charge. Ask for a refund of your macbook or a replacement unit.
You spent money on something, you want it to be in good condition, and since its under warranty you should utilise it legally and they should oblige to their responsibilities.

Juz my 2 cents.