Turns out it is a City Ordinance for pet owners to clean up after Fido and Felix - well, Fido, anyway.
From the Animal Control page of Boston's website:
The City of Boston's dog fouling ordinance, section 16-1.10a of the city code, requires dog owners to remove and properly dispose of any feces left by their dogs. This ordinance covers waste left by your dog on sidewalks, streets and parks, and on your neighbor's yards.
When walking your dog, the law necessitates you to be prepared for such occurrences and be equipped with a bag or some other means of feces removal. After it is removed, it is important to dispose of the feces properly, either in a toilet or in a trash container (after being secured in a plastic bag).
These laws are intended to protect the public from potential hazardous health conditions, and prevent environmental pollution caused by dog waste.
A call about feral cats yielded nothing helpful.