The TSA was created by the act of Congress, which also empowers it to make and enforce regulations, and gives the executive branch the authority to press charges. In addition there are also FAA regulations which are also authorized by the Congress.

Acting against the TSA/FAA regulations is acting against regulations placed by an agency authorized by the Congress to place it. It makes it illegal.

Consider "Speed Limit" sign - there's no law for every street and road limiting speed on that particular road. But there's a law allowing designated agencies to put and enforce speed limits on various roads based on various conditions. TSA regulations are similar in nature.