Placing unauthorized devices or advertisements along a highway is a
crime in Massachusetts, although not a very big one - it's subject to a potential $10 to $100 fine plus recovery of the cost of removing the devices or advertisements. There also are more stringent
laws against defacing public or private property - but since the devices (at least in NYC) were only attached to metal by magnets, it's not clear that any property was defaced.
I suspect Turner will be more than happy to pay Boston for the cost of removing the devices. They've received more than $700,000 in free publicity as a result of this stunt.