The following refers to BP activity where they routinely flout the law within the 100-mile zone :
For instance, Border Patrol claims the authority to operate immigration checkpoints. Agents, nevertheless, cannot pull anyone over without "reasonable suspicion" of an immigration violation or crime (more than just a "hunch"). Similarly, courts have determined that outside of Ports of Entry [Border Patrol cannot search vehicles in the 100-mile zone without a warrant or "probable cause" (a reasonable belief, based on the circumstances, that an immigration violation or crime has occurred).
Outside the 100-mile zone, how is the act of boarding Greyhound buses to ask for documentation anything other than unconstitutional?
It's completely unconstitutional. But as I said, courts have ruled that law enforcement agents have the right to lie to you in order to get you to waive your rights. It is, BTW, a crime for you to lie to a federal official about anything at all.
That's the country in which we live. It's up to each of us to affirm our rights. This woman did something very brave but very dangerous. The good part of this is that her loud and public assertion of the rights of everyone on that bus was negative reinforcement; those agents will be a little less likely to go through that kind of public calling-out again.
The ubiquity of video cameras out there make them a little more cautious about abusing people. Not all of them do this kind of thing. But being exposed and embarrassed is a good thing, albeit personally risky.