I thought the same thing when I found out about that, but it kinda makes sense.

Facebook tries to show you the most "interesting" posts. If you are a member of many groups, there are probably too many posts in your groups to show all of them in your newsfeed. So Facebook just picks whatever they think is interesting.

Or the poster can pay to have everyone see the post.

Anyway, I prefer this monetisation strategy to creepy tracking.