Dewey Square is not on any tourist route. It's not at any shopping center. It's beside one of the busier commuter rail stations. Bostons office workers are the most affected by the smell.
You’re right though someone called up Howie and mentioned the bit about the city not wanting them there for First Night at least...even if there were to be no events for First Night there (not sure if there are) some people might go near there and get a negative impression of the city. Similar to how cities that hold political conventions bus the homeless/
vagrants off somewhere else for awhile and try to clean up
litter, etc.