Reading in the thread view isn't a very good way to follow the topic progression generally anyway, so this pushes people towards the more organised post-by-post method. You can, of course, read all of the posts in thread view and then "skip" those posts to mark them as read.

I personally believe that it helps prevent people from accidently ignoring posts that might answer questions they would continue on to post, etc., but you'll have to wait for [Hedda]'s take on it.