The tripod law in DC might be listed under federal, not DC. When I was in college (several years ago), I went to DC while looking at grad schools. My uncle warned me that I couldn't use a tripod near "the Mall" or any of the monuments. It actually has to do with safety and someone tripping over the legs and also with mistaking one for a weapon. Same rule in Museum of Fine Arts in Boston.
In law enforcement overall, there's more of a sense of "I can't let something happen on my watch." They'd rather err on the side of overcaution than potentially let someone get away with something. Are they right to do it? Maybe, maybe not. But it's absolutely impossible for all the assorted federal, state, and local law enforcement and security guards to strike the right balance that will make everyone happy. The political climate in Massachusetts is a little different from places like TX, but I bet the law enforcement reacts similarly to someone with a tripod at a bridge.