Yes, it should be legal to restrain airline passengers if they are intending harm to themselves or others while aboard an airplane. Since they are in a confined area, the potential for harm to others is greater and drastic measures may need to be taken, which could include restraint and other physical interventions.
There aren't many other options at 35,000 feet. If someone is clearly a danger to the crew or other passengers, there is no reason not to restrain them. The option of removing them from the situation does not exist, ,and it's unfair to expect crew members and other passengers to be endangered.
When you are in flight the options are greatly diminished. If a passenger is causing a risk to the other passengers or the entire flight then they have to be stopped in some way. Restraining an airline passenger may be the only thing that can be done to keep the situation under control and all the passengers safe.
Just because a person buys a ticket does not mean that they are entitled to whatever inappropriate behavior there may be. One or a few people may be endangering the lives of the rest of the people aboard a plane or disrupting departure, or they may have a weapon or wish to do bad things. Your rights don't go out the window, but everyone else has rights too.