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.