It's deliberate.
We had one of the 'usual idiots' around recently, who enjoyed repeatedly clicking on the 'Submit' button without typing anything. So long strings of 'replies',consisting of nothing more than the words 'Type your answer here'' were appearing in lots of threads. (there were also a few people who were accidentally clicking on 'Submit' too early, again with 'Type your answer here' appearing on a thread).
So the Techies have now modified things so that something has to actually be typed into the reply field before the 'Submit' button becomes active.
Chris