Mocking the Ethicist

I have nothing personal against Randy Cohen, and in the few interviews with him I’ve read he comes off as somewhat self-deprecating. While appearing not to take himself too seriously makes it harder to criticize him, there are sufficient problems with his most recent column that I cannot resist. Plus, it provides more than sufficient material for my first post.
A correspondent writes him: I was disturbed to see a man looking at pornography on a New York Public Library computer. When two children sat down near him, I decided to take action, but he instantly switched to an inoffensive video game. A security guard told me they were keeping an eye on a couple of people acting similarly and would catch them in the act eventually. Should I have alerted someone as soon as I became aware of this man's activities? Anonymous, New York
I do not want to devote much discussion to what 'act' the guard was waiting to catch the man in other than to say that Cohen presumes throughout his response that the only act being referred to is viewing pornography, and I will make the same presumption.
Cohen begins his response:
A guard monitoring someone's reading? That's not my idea of how a library operates. Libraries should provide for the free exchange of ideas -- not just ideas you or I find palatable, not just ideas suitable for 5-year-olds.
So far, this response has the twin problems of being false and non-responsive. It is false for two reasons. The OED meaning of ‘exchange’ closest to Cohen’s usage is: An alternation of statements or responses in the course of a conversation; an interlocution. Also (usu. in pl.), a conversation, dialogue, or argument.
This envisions an exchange as multi-directional transfer of information. But surely the purpose of libraries is (primarily) a unidirectional transfer. So it’s false that libraries should be an exchange.
Further, while I can't really make sense of what Cohen means by 'free' in this context, I assume he just means that this exchange shouldn’t exclude disfavored viewpoints. But any given library only has limited resources. The library of any given community is (and should be) free to make choices about what books and other media to purchase. But this will not lead to a free exchange, since the community will not purchase books which contain ideas the community does not want disseminated. I can imagine a reasonable minority rights objection to this, that there are situations where less than a majority of a community wants a particular resource available in the library and yet it should be acquired. But that is irrelevant; my goal here is not to discuss the correct process behind library purchases, since I know nothing about that topic. I am not suggesting that it is better for libraries to be un-free, but rather that free or un-free don’t enter into the discussion. Cohen's claim that libraries should provide for the free exchange of ideas, a claim which motivates his whole column, is utterly baseless. The purpose of libraries is not a "free exchange," being neither free nor an exchange.
Cohen continues: And librarians should not be forced to censor patrons' reading, let alone eject them for looking at disturbing images.
He is just cheating here. The issue raised by the correspondent does not touch on forcing librarians to do anything. Cohen confuses the permissibility of an action with it being obligatory. Are libraries or librarians permitted to restrict the viewing of pornographic material? Later in the column Cohen speaks approvingly of steps taken to do this. Yet if he approves of this, the above quoted line is arguing against a straw-man. Also, disturbing and pornographic are not coextensive.
This step by step argument is taking too long; hopefully I've already illustrated Cohen's faulty reasoning. I hope this post will not be interpreted as being in general pro-censorship or anti-pornography. I probably should not have used forms of the word pornography in this post; it could lead to it coming up in the wrong searches.