It could also just be a deleted post. So a whole topic isn’t necessarily going to disappear.
In my experience people tend to read the deleted message before it disappears. Sometimes people delete messages when they think more on what they just posted, but if the delete history is there people will still read it (allowing more people to read the hateful message, if that’s what it was). Maybe the posts could still be made visible to staff, but not regular users. 
I’m not necessarily asking for the functionality to be changed by the way, I’m just curious what the reasoning was.