You’re paying for a service that they provide. Because of this, you are on that airline’s property (the plane). They have the right to remove you at will. Forcibly is up to airport security. An airline doesn’t choose to wrestle an unruly passenger off, that’s what the airport security is for. All in all, if an airline chooses you to be removed from an overbooked flight, don’t try and fight back, just take your voucher and get off.