There's a difference between "secrecy" and "security", and when people complain about how releasing some information would hamper / endanger lives, they mean that some amount of secrecy is involved in protecting stuff.

Computer nerds often don't understand Kerckhoffs's law this way. The security of a system should not be dependent on obscurity, but some amount of security will depend on secrecy, i.e. your private key / password.

So for example, the list of all confidential informants is kept secret. The release of that list of informants would "hamper the department's ability to conduct operations and endanger the lives of New Yorkers", in addition to endangering the lives of the informants themselves. Does that mean that it is dependent on "obscurity", or, secrecy? Yes, but that is the way it always is. There will be some information that if disclosed could result in bad things happening. "Education in science" won't help with this basic fact about the universe.

Of course, whether or not these vans genuinely represent information that should be kept secret, who knows. Often people believe that too much information should be kept secret, and when you're talking about information in a free society, we probably should accept some danger over granting someone power, but that seems to be a minority opinion...