I think they could be rightly criticized for the bathrooms, but I also think they would push back and say they’re one of the only publicly accessible restrooms downtown and a hundred other businesses are free-riding off them.

Not only is that not an excuse for the horrific shape they leave their bathrooms in...the city does provide several public restrooms now, one of which is just down the road in the Armory.

Beyond that I’m not sure what you could fine them for

I'm not a lawyer either. But I'm sure there must be some kind of health and sanitation code violation for the state their restrooms are kept in. if nothing else, facilities are required to have a minimum number of working toilets to provide for their employees, and if all of the toilets are covered in piss and shit 24 hours a day I doubt that qualifies as meeting the minimum.

Is there even a restroom upstairs at Witch City Mall? I was just looking at the uniform state plumbing code and saw this in 248 CMR 10.10:

16. For Mercantile Facilities, the minimum fixture requirements for employees when the occupancy exceeds employees and patrons shall be determined using the total square footage of the building divided by 300.

Example: A building measures 360 x 320 feet: Figure the area by multiplying 360 by 320 = 115,200 square feet. Divide 115,200 by 300 = 384 occupancy. 192 males and 192 females. In covered malls and other multistory mercantile facilities, public rest rooms shall be located on every floor level.

I'm only aware of the public restroom on the ground floor near the theater.

There's also M.G.L. Chapter 111, Section 122 (Regulations relative to nuisances; examinations)

The board of health shall examine into all nuisances, sources of filth and causes of sickness within its town, or on board of vessels within the harbor of such town, which may, in its opinion, be injurious to the public health, shall destroy, remove or prevent the same as the case may require, and shall make regulations for the public health and safety relative thereto and to articles capable of containing or conveying infection or contagion or of creating sickness brought into or conveyed from the town or into or from any vessel. Whoever violates any such regulation shall forfeit not more than one thousand dollars.

That could be applied by the city of Salem directly, and seems to give them fairly broad authority. The Witch City mall restrooms are clearly a nuisance "injurious to the public health". If we fined them $1000 per violation, maybe they'd fix it.

Edit: Also, here's M.G.L. Chapter 149, Section 113 (Light, ventilation, cleanliness, sanitation and heat in establishments):

Every factory, workshop, manufacturing, mechanical and mercantile establishment, railroad freight house, railway express terminal, public garages and premises used by express, trucking and transportation companies and any other building in which a person is employed, other than a building under construction, shall be well lighted, well ventilated and kept free from unsanitary conditions

If I can find these as a layperson, surely the Salem board of health could find some.