The pool is in fact part of the Spa. The charge $15 per day is for use of all the spa facilities - louge chairs, steam, sauna, etc. As posted this has been in place for at least 3 years. The problem in the past the quiet relaxation of the spa area was turned into the indoor party area.
If the pool is important to you sign up for the cruise long spa pass at a discount. In the past you could not have used it for laps with people just standing in pool yapping with their friends. Keep in mind the pool is not very large and it would be perhaps four or five strokes per lap. Check it out when they have the open house the day you board and decide if it is worth it to you.
I was on the Dawn two years ago and they were charging $15 per day or $60 per week in order to get a pass to the "Quiet Area" of the spa where the lap pool is located. It is a small pool by any other standard but on a ship, it is one of the longest that I have seen. I did my laps daily in the pool and it worked out well except that I did find one end of the pool to be almost too shallow since I would hit my hand on the bottom when I was swimming. I have found that the other sister-ships that NCL has with similar spa pools charge the same or more for using the pool.
The previous comment is this thread is correct, many times there are people standing in this pool talking since they do not understand that it is a lap pool or they cannot read the sign that clearly states so.