Wow! I've never heard of a dive light being taxed. If I ever return for a dive trip in the Sea of Cortez, I will never fly to Cabo again. I'll fly into La Paz or Loreto.
Just curious, did they pay the tax with cash or credit card? My thought would be to dispute the charge to the credit card company as being a fraudulent charge with forced seizure of the passport until payment was made. I know you're not the U.S. and the laws are different, but it may be worth a try.
This really disgusts me about Mexico.