They're taking a leaf out of the airline playbook and squeezing everyone in like sardines to make more $$$ it looks like
Ah, airlines, finally something I can speak to. Over the past 20 years, the airlines have made massive improvements to their revenue management systems, which is simply the algorithms that tell them how much they should charge for tickets. And when I say "improvements," I mean, of course, that they've figured out how to charge you more.
The Celtics are doing the same thing, although not with the same level of sophistication, since they can only change prices once per year and can't take seats out of the stadium (or add them) when demand rises or falls. Thus, the decision that they're making is based on a snapshot view, and their picture looks awfully pretty now. Every seat is taken, the team is okay and they've got a waiting list with thousands of names of people who are willing to snatch up those seats if I don't want to renew.
The bet that they're making is that the list of names will be there for a while (probably true) and that there are enough people who are okay with the smaller leg room that they'll still be able to sell out (also probably true).
In effect, they raised prices multiple times this year. They raised the ticket price but they also reduced your "product" by taking away some leg room. And for some of us STMs, we got worse seats because of the effects of the renovations.
And yet, I keep renewing each year, so they've probably got it pegged correctly. When I call my STM representative to complain, she tells me that they only raise prices to put the money back into the team and get us the best product. I'd almost be happier if she just said, "We raise prices for the best of all reasons: because we can."