I was booked on a flight for Chicago from Bangalore (India), I have a B1 visa. During check-in at the airport counter (for air India) they guy (who generates the boarding pass) said he gets an error message that says 'DO NOT BOARD - CONTACT US TSA/CBP' can anyone please tell why this is...
The CBP/TSA have recommendations from both their own and external parties about certain individuals for whatever reasons - innocuous, cautionary or other, indicating that their recommendation is that said individual not be allowed to board a flight.
For example, the CDC (Center for Disease Control):
CDC provides “Do Not Board” recommendations to CBP and the Transportation Security Administration (TSA) regarding individuals who may be infected with a highly contagious disease, present a threat to public health, and should be prevented from traveling to the United States via commercial aircraft. TSA is performing vetting of all airline passengers coming to, departing from or flying within the U.S. to identify matches to the “Do Not Board” list and flag matched individuals’ records in the Secure Flight system to prevent the issuance of a boarding pass.
So at this point, you do what the note tells you - you (hopefully with the ground staff) contact the TSA/CBP, determine the reason, and see if there's anything you can do to get around it. For example, the original "No Fly List" generated a lot of false positives, especially if you say, had the same name as a suspicious person. If you were erroneously put on the list, you can appeal it, but it can be a lengthy process.
I am pretty sure the message 'DO NOT BOARD - CONTACT US TSA/CBP' means 'DO NOT BOARD - CONTACT US TSA/CBP.'
It might be an immigration issue, and they were not willing to let you enter into the US anymore, despite your B1 visa. Has your life situation changed lately?
Or it can be a safety issue, and they thought that for whatever reason you are a risk for air transportation. That is, your name is in the no fly list.
To get a US visa, one must establish a non-immigrant intent which is a strenuous task itself.. A visa does allows a person to enter the United States and THEN ask permission to an immigration officer to be admitted into the US territory(As you've said). The problem here is that this individual is not even allowed to travel. It also seems like that he is refused boarding because he's on a no fly list. It's most likely a false positive case, otherwise, the embassy would not have issued the visa. If I was you, then I would contact the consulate to clear off this matter.
Also, try to contact the CBP or email them at this address to see if they reply.