It is supposed to simulate a small amount of diffusion by the lens and possibly other reflections within the camera, and these are optical effects don't change according to luminance. So it works like this by design. Perhaps bloom isn't the best name for this, as the term bloom can mean other phenomena that happen at the film or sensor which do change according to luminance, and which Terragen doesn't do.
I think 0.1 gives a stronger effect than you'd see with most cameras, but I wanted the default to be high enough to cause a noticeable difference when you first enable it.
Most cameras also push the shadows down a little bit more than TG's default tone mapping, so combining TG's bloom with some levels/curves adjustments in post (or increasing contrast in TG) sometimes looks pretty good.
Our 2018 roadmap includes realtime previews/adjustments of TG's post effects and tone mapping in the Render View.
Matt