The purpose of your trip is actually valid. It can trigger certain customs limitations on monetary value that you can legally return to the country with. It's obtuse, but it's a valid question. It's also important to note that you have zero civil liberties, even as a citizen, within 100 miles of a border to the United States, of which an airport has been considered I've in some cases.

TSA security theatre on the other hand is a whole different ball of wax. Very little of what they do contributes more to security than prior measures or alternative, less intrusive measures. The fact that programs where you can pay to avoid it exist prove this point. They already have the same level of access to every person thorough the dragnet of law enforcement and intelligence databases that every single air traveler gets processed through. When it's trivially easy to spot numerous gaps in security in every single airport I've been to, it's more a statement to the fact that most people are good than it is to the fact that we're good at catching bad guys. An airport just needs to be a less viable target and bad guys will go elsewhere that's easier.