Part of the issue with it wasn't the concentrate itself, but the sheer number of people being arrested for trafficking by trying to bring it back into the country.

One of the things people tend to forget is that even when something is decriminalized on a state level, it remains illegal on a federal level unless Congress passes a law that over rules the previous law.

The laws which govern customs, which handles importing of goods into the US, is federal law, not state or local. It also covers things like tobacco, and food. Meaning yes, you can also get arrested for bringing too many cigarettes into the country, but I digress.

The problem that the US noticed, was that US citizens were going to India, buying charas, and then trying to return to the US with it. When they would pass through customs, those people were getting found out and were being arrested on smuggling or trafficking charges. Since these were federal charges, this was happening in places where cannabis use was technically legal, and it was creating a severe backlog in the federal legal system.

To help curtail this, the US State department reached out to other nations where this drug originated, and requested that they do something about the sales of it in and around where major international airports were. The reasoning there being that it would prevent US citizens from getting it and bringing it back; while at the same time it would help cut back the number of native Indian smugglers, since drug mules rarely operate too far from points of embarkation due to how the internal (inside their body) smuggling is performed. Meaning there's a short time frame between ingestion of the smuggling packets, and when the mule boards a plane.

For India, there were concessions also made, as things like this are never so one sided as they're presented on places like Reddit or Facebook. These concessions were a relaxation on import and export tariffs (basically taxes), which would allow legal importation of other goods at a higher rate, since companies wouldn't need to pay as much to do so. Given how high tariffs are to begin with, when offered a chance to lower them and only needing to make a simple concession to prevent sales of something around major airports, well it's no surprise that the Indian government agreed to the decision.

Oh, and something that may interest you. Going back to the point I made about federal charges, LAX (Los Angels Airport) has clear warnings that while you may legally have cannabis on your person inside the non secure areas of the airport, attempting to go through TSA with it will result in your arrest for smuggling charges.

Edit: One final note. TSA and Customs are not the same thing. Customs officers, those who search the bags of returning passengers, are federal officers. TSA are employed by the airport themselves, or in some cases by the state.