Columnist: If you received an unsigned letter, you would likely have some doubts about the truth of its contents. But news stories often include statements from anonymous sources, and these are usually quoted with the utmost respect. It makes sense to be skeptical of these sources, for, as in the case of the writer of an unsigned letter, their anonymity makes it possible for them to plant inaccurate or slanted statements without ever having to answer for them.
The statement 'pointing out that a certain attitude would presumably be adopted in one situation, in order to support the claim that a similar attitude would be justified in an analogous situation' is the way in which the columnist's argument proceeds.