Link: http://www.nytimes.com/1994/01/23/nyregion/soapbox-a-little-...
Link: http://www.nytimes.com/1994/01/23/nyregion/soapbox-a-little-...
http://iquantny.tumblr.com/post/144197004989/the-nypd-was-sy...
I guess that's open to abuse with people painting active hydrants to make them appear inactive and open up more parking spaces.
The covering of them mentioned by nommm-nommm might actually work, though.
If marking them would generate any revenue it would be done very quickly. As it is not marking them generates tons of parking ticket revenue, so that's what's done.
If you don't enforce the parking law for the non-functioning hydrants, people may become complacent or confused and start parking in front of necessary ones.
But removing them isn't worth it either, because it's expensive, and there's no particular need to. (The article says they can afford it, but even so, there are probably better uses for the money.)
How about creating legal parking places? Obviously some of those parking places were heavily desired.