Couple of points;
1) a fine won't work, because people will take their chances and try it anyway.
2) how will these fines be enforced? You will need cameras in the cabin because I would doubt that eye witness testimonies during the chaos of an evacuation will hold up in a courtroom.
3) better IMHO would be a legal approach in the direction of "endangering people's lives" or "obstructing an escape route" with actual prison terms.
4) perhaps blacklisting passengers, banning them for life to fly on any IATA airline?
5) if none of the above work, I guess we can always (re)introduce the death penalty...
