My guess is that this is a negotiating technique to apply pressure on somebody to cut their price. After all, if they wanted to cancel it, they would (as long as they haven't committed to pay someone to build it). Most likely it's to tell someone "Get reasonable, because we can still cut the whole thing out." whereas as long as the contractor believes that the MBTA can't back out of it, they have no incentive to reduce the price they've quoted.