If you read the article (you should) you'll note that the shops with the concession do not have the freedom to arbitrarily set such high prices (which would make it that simple), and that the Port Authority who is supposed to ensure that prices adhere to the set guidelines, is apparently not doing its job and is withholding documents that can proof this despite these documents generally being considered open information for citizens to request.

The issue discussed in the article is that the NY Port Authority has regulations to clamp the prices of these items to comparable items in NYC (based on the average of the 3 lowest cost comparables), and they are not being transparent on which comparable items (if any) they are comparing to.

So no, it is not just a simple issue of supply and demand, it is an issue of lack of governmental transparency.

The article mentions that according to the Port Authority rules, vendors can't charge more than 10% of the street price of the equivalent products. So no, you can't charge whatever you want just based on supply and demand.

The article is trying to understand how they determine the base price.