Teacher: Journalists who conceal the identity of the sources they quote stake their professional reputations on what may be called the logic of anecdotes.
Conclusion
Journalists stake their professional reputations on the logic of anecdotes when they conceal the identity of their sources.
Evidence (this is so because)
The statements reported by journalists who conceal their sources' identities are dissociated from the precise circumstances in which they were made.
These statements will be accepted for publication only if they are high in plausibility, originality, or interest to a given audience.
These are the properties of a good anecdote.
Evaluate
This argument is not a direct fit for any of our common patterns. However, it does seem to have an important New Concept in the Conclusion: "staking their professional reputation".
We hear about what it takes to get an anonymous quote published (it has to be plausible, original, and interesting) and that anecdotes share these qualities.
And then we take a leap to "staking their reputation on the logic of anecdotes".
It's possible that even though anonymous testimony must have some qualities that are shared with anecdotes, there are still important differences. It's also possible that whether or not a journalist can get the anonymous testimony published has little to no connection to their professional reputation.
Goal
Lacking any specific idea of what we want, let's try to buttress the argument by making anonymous sources even more similar to anecdotes, or by filling in the gap about why this affects a journalist's professional reputation.