Releasing gas in sufficient concentrations that would put and continue
to keep adults asleep during the emotional trauma of a hijacking would
likely prove lethal for infants and small children. Also, if a
hijacker claimed to have a bomb detonated by a release switch, putting
him to sleep would detonate the bomb. And, of course, hijackers need
only a gas mask to defeat the "la la land" plan and, with the passengers
asleep could concentration their efforts on getting into the cockpit or
destroying the plane.
There are times when things have to get a little bloody.