Having a few of my soldiers sign on for the $15 grand when we were in theater as well, I did a little checking up on this with my Retention NCOs, and also got copied an email via the Washington State Command Sergeant Major from the Washington Retention Officer in Charge.
Basically, this reneging doesn't apply to 99% of the soldiers. Any reservist called to full time and sent away get their $15K. The catch is the reservists that were already working full time, in effect as Cadre. (Active Guard/Reserve or Federal Technicians, depending on job).
My guess is that the retention people in country had no way of determining when everyone was on active duty who was AGR and who wasn't, and just gave the same forms to everyone. Personally, I think they should probably just pay out the cash and learn from their mistakes, even if other soldiers in future don't get the benefit of that particular re-enlistment bonus.