TSA has some pretty strict rules about what you can bring on a plane, right down to the amount of liquids in your cabin bag. Seasoned flyers are well aware of these rules by now and it’s understandable that new flyers could make basic mistakes.
However, some people love to push the boundaries when it comes to these things, and somehow think they can sneak ridiculous or even dangerous items or even living things past TSA. This can result in legal consequences that can end up costing more than the flight and the restricted item combined.