Knife Owners' Protection Act of 2021 This bill permits an individual to transport a knife between two places (e.g., states) where knife possession, carry, or transport is legal. The knife must be securely stored during transport, unless it is an emergency knife designed to cut seat belts. An individual who is transporting a knife in compliance with this bill may not be arrested or detained for a knife violation unless there is probable cause to believe the individual failed to securely store the knife during transport. An individual may assert compliance with this bill as a claim or defense in any civil or criminal proceeding. The bill repeals provisions, commonly known as the Federal Switchblade Act, that prohibit the introduction of switchblade knives into interstate commerce. It also repeals the Ballistic Knife Prohibition Act of 1986, which prohibits the possession, manufacture, sale, or importation of a ballistic knife. Finally, it eliminates two criminal provisions—one that restricts the mailability of switchblade knives, and one that restricts the mailability of ballistic knives.