In the post 911 world, how could you not be weary of a possible hijacker on a plane? Also, with the recent disappearance of the plane in Malaysia. If I encountered someone physically overpowering another individual or threatening to on a plane I would have to act and overpower them. I would like to note that I would never subdue someone just for being rude or looking and acting in a way that I do not like. I would only have subdued the passenger if he was actually threatening someone's life.
Yes, it is justified to restrain or subdue an unruly passenger, it is my belief that passengers on public transportation enter into an unwritten contract of basic decorum that involves behaving rationally.
An unruly and irrational passenger puts at risk other passengers and the pilot, driver, or conductor of whatever mode of transportation they are using, if a person finds themselves in a situation where they cannot control themselves they should leave the vehicle, if they cannot remove themselves from the situation then they have to be subdued for the good of the majority.
There is a fascinating take on airline security that, rather than there being an increased risk of terrorism since 9/11, the same will not happen again because passengers would intervene in the event of danger. If there is serious danger that could potentially end in death, it is important for people to intervene.