Umm they didn't raise ticket prices last off season. This is the 1st time in 2 years. I would wager there aren't many clubs who did this.
http://www.titletowntix.com/red-sox-will-not-raise-ticket-prices-in-2009
And if people want to keep Fenway around, they are going to have to pay for it. Hence not backing prices down.
And in what other business would the supplier back off on his prices when he's selling out all the time? That view is just absurd and a good way to run your business in the ground. The only way the will drop prices is if no one shows up. And guess what, it's not happening.