Don't see what is so difficult to understand here, TBH. A club may have captive fans, but it doesn't prevent it from being subject to the rules of supply and demand on some level, which Arsenal is.
The way you talk you would think that football as a whole doesn't exploit its fan base - only AFC does. That's bollix, as you well know. Football sold its soul years ago, and if AFC wasn't trying to maximise its profits it would be regarded as commercially naive - in fact that's exactly what many were saying before we finally gave in to the lucrative pre-season tour model.
...and FWIW I disagree with Letters' point:
If as an Arsenal fan you don't like ticket prices, you can stay at home. If you don't like the food or drink prices, eat/drink outside the ground.You're an Arsenal fan, you can't go elsewhere.