I don't know that one can appeal to logic any more than one can claim something is true because it's "common sense". 
It IS "public" display of herself but the public as you call them are workers, not like any of the free public who walk our streets. In government agencies there is a dress code, so anyone can be in violation of that. If work is public, why have any dress code except for safety?
No one is objecting to looking at a woman who has a short skirt of cleavage. But one cannot take their picture without consent where they have a reasonable expectation of some level of privacy. Look it up. If a woman had a short skirt, would it then be OK to take a picture of her bending over the file cabinet?  Looking is not the same as taking a picture. 

You may have to just accept it as the current law. In France women can wear veils. That's weird to me.