> I'm having my trouble getting my head around this. That seems to say that a city WOULD benefit from the stadium because people would spend their money in a different city. I'm clearly not understanding it.
No, you're reading it backwards. It said:
>> tourism does NOT see an increase as the result of a sporting event
Which is a bit wrong in using the word "event". Sure, city revenues will increase from a home game -- but that evens out when local fans travel elsewhere to see an away game, and spend money there that they otherwise would have spent at home. OK, it works if you take "event" to mean a set of home + away games between two teams... But anyway, having a team that attracts some away fans and their money will logically give no net revenue increase to any one city, since presumably (and on average across all cities, as a mathematical certainty) all away fans spending money "here" are also local fans somewhere else, not spending that away-game money in their home town.
> For example, I'm thinking of all of the people from Maine and NH that come to Boston to watch the Bruins play hockey. Without the team, they'd most likely not go to Boston, but might go to Manchester NH, or Portland ME instead. That's definitely not zero sum for the city of Boston or state of MA.
A) So Bruins fans only watch home games? OK, maybe they're more travel-averse than fans of other teams in the league... But one would think most cities' fans are about equal in this respect, so on average just as much money is siphoned out of the greater Boston area for away games.
B) See that "greater Boston area" above? That's your "city", for the purposes of this discussion. That the "sports-fandom city" happens to stretch across municipalities and even states in your case is kind of irrelevant; in principle it's no different than rivalry between various districts within a city about where to put the stadium. Wherever you put it, of course the immediately surrounding area gets more of that revenue than areas further away. :: It is zero-sum for "the city of Boston" as defined by Bruins-fandom.