The important bit: > he may have been singled out for a search because he was the only black person on that part of the train.
The key part for me is right here - "A DEA agent boarded the train at the Albuquerque Amtrak station and began asking various passengers, including Rivers, where they were going and why." Yeah, no. We've already crossed into unacceptable police behavior right there. There's no reason to suspect me of a crime, so you don't get to question me. At all. Forget whether or not it's smart for me to answer it, it's wrong for…
What is it when the laws only apply to you but not the government? In this case the law is specific to people doing the violation "under color of law".