I think it's probably just to support their offices and employees located in Chelsea. They probably had to have some agreement with the city to have an open net

I think it's probably just to support their offices and employees located in Chelsea. They probably had to have some agreement with the city to have an open network if they wanted to put up antennas for their own use, or maybe they just figured--being the do-no-evil company that they are--that if they were canvassing the area with their own wifi that they might as well have a parallel network open to the public as a public-relations stunt.