I think you are misunderstanding the way that U.S. airports work and how American law affects them.
Most major airports are run by the local municipalities. Therefore, they are public spaces. They have very specific restrictions that mostly affect operations, but, by and large, the American public is free to wander into an airport and shop, grab some food, sit and people watch, take photos, etc. with no expectation that they have to be actually flying or even in possession of a ticket or want to purchase a ticket.
In fact, many airports make big deals about things they have installed at the airport and encourage the public to drop by. PHX has a tiny, but lovely art museum in T4. Several years ago, one of the terminals had a display of Kachinas on loan from the Heard Museum. When T3 was initially opened, it housed an upscale restaurant on the concourse leading to security and the gates, and the City of Phoenix encouraged people to come to eat at the restaurant even if they weren't flying or had any other business at the airport.
Even LAX turned its old Theme Building into a
funky restaurant. (Go for the drinks as the food is marginal at best.)
But our public airports are not regulated by laws or rules stating you have to have business to be there. You go if you want and you leave when you want (subject to closures, etc.).
So, now we get to the part where the OP was taking photos inside the terminal. Again, there is no law stating that you have to have business at an airport in order to be at the airport. Wander around all you wish. There is no law against that.
Additionally, there is no law against taking photographs inside an airport terminal. The U.S. Supreme Court has also said very clearly that the minute you step into a public place, you have given up any expectation of privacy. That is completely logical. You cannot be demanding that people avert their eyes when you enter a public space. People will see you and you made the decision to leave the private confines of some private property. So, the OP taking photos did nothing wrong according to the law.
Now, we turn to the issue of suspicion. I had written a long bit about why we shouldn't be automatically suspicious of photographers taking photos of the security area and TSOs, but then I read a quote on another threat and on the TSA blog and saw a reference to a post made by Bruce Schneier, one of the most knowledgeable individuals on security in the U.S. I will quote part of it here and strongly encourage you to
read the rest.
So, I ask again: why is taking photos of the security area and TSOs suspicious?