DO NOT EVER buy a ticket with cash. It's silly since it's the nation's currency.
Yes, it is silly you can "avoid it" by other card methods. But at least theres a longer trail associated with that, and probably puts you at a lower risk of being labeled as a concern.
In the early 2000's (pre- and post-9/11) my wife had a job where she would 2-3 times a month get a phone call at 5am and then have to go to the airport and get on the next plane to whatever city was specified by her boss, take care of business, and then return at the end of the day. Paid for her ticket in cash, and her only luggage was her purse and her Palm Pilot (actually a Sony Clié UX50, which I remember because it was awesome).
She got pulled aside by security every single time for extra pat-downs and such. It got so frequent that they knew her by name, but they still went through the routine every single time.
She's very glad not to have that job anymore, but for other reasons.
Yes, it is silly you can "avoid it" by other card methods. But at least theres a longer trail associated with that, and probably puts you at a lower risk of being labeled as a concern.