If you want simple data, lets look at the price of bottled water or soda at an airport (I usually bring an empty with me but not the point). It is priced the freakin same across every eatery. No collusion?
There’s no collusion if they’re all owned by the same vendor, which is common (and pointed out in TFA). The problem then isn’t collusion; it’s that the agencies responsible for ensuring fair airport pricing are captured by the businesses that they’re supposed to regulate.
Go to any airport an Asia and you will find plenty of drinking water fountains. Sure, sometimes the cold water is broken but at least it's sanitary drinking water.
Meanwhile Frankfurt tells you that the tap water in the restrooms is safe to drink. Absolutely absurd that Airports where you are effectively stuck for possibly hours and are paying for that priviledge can get away with not having to provide basic neccessities.