My guess - contractual obligations or some other liability?

What benefit would the author get from naming the agency involved?

There’s a big legal risk involved in doing so and no upside that I can see.

I was disappointed the author didn’t have the guts to name them.

I would hope he could get some satisfaction in knowing that he might be able to stop them from taking advantage of clients in the future.

Easy for an outsider to say, though I’d imagine it’s a perfectly reasonable precaution to be cautious in the first post they make. Also he’d then not need to worry about what he exactly says in the post or mince his words/self-censor.