.... Well first off that's racist.
2nd Being an American citizen doesn't make you special, you are just as likely to be a threat to everybody.
You can easily have drugs or other illegal substances hiding in the lining of diaper, people will go through great lengths to smuggle something.
Whether you're IC or not you can be hiding something, wearing a soiled diaper is not going to stop them from checking to if you're hiding something and using "I'm disabled, don't embarrass me card;".
And if they think that there's something being hid in your wet, messy or dry diaper, they have the authority, you can be hiding a dangerous chemical agent, especially if strongly resisting their demands.
They care about ensuring the safety of people's lives, not your feelings getting hurt from embarrassment.
So your, "I'm an American Citizen" means nothing, Americans can be terrorists too, as can any other person from any country, anybody everybody is a potential threat and that's how they view it.