I see what you mean. In that case I think they're just trying to fill out some of the whitespace in that view. However, if you reduce the browser window horizonally, the avatars collapse down to just the last user to post in that thread.
I really like the whole Discourse concept. The people behind it seem to know what they're doing. You can take a look at their Features page for more info - https://www.discourse.org/faq/ It's not perfect of course, but they'll be the first to admit it.