Here's a simple breakdown. Number 1- the mother must know she does not have the RIGHT to fly, and therefore is subject to any whims the TSA has. She is consenting to the act by merely flying, if she doesn't want to be searched, no one is going to force her to be, all she has to do is leave. Flying is not a right. This is not a message in defense of TSA, it is merely fact.
This is why she got convicted. Due to her behavior, it clearly made TSA nervous. And when people get nervous, they have to, for security and safety reasons, devout more personnel and time to that person. Because of this, they had less resources to search and secure all the rest of the passengers, any of whom who could be wanting to do harm. This mother, likely knowing what was going to happen once she got there, and though she has a right to be upset, caused the TSA to be distracted from their duties. She chose to act in such a way that the process was less secure for other people. Even if she only delayed 1 other person 1 second, she committed the crime she was found guilty of. And she would have. If you watch the video you can see how many agents they had dealing with her, and how calmly they were trying to do it to avoid such a situation.
BTW, she only got a slap on the wrist, which is deserved and far less than what she could have gotten.