They're the "Stinger" seats, available to season tix holders as an option of about $5/game - basically temporary seat back/butt cushions that bolt on to the bleacher seats. I wouldn't recommend them - they force you to sit unnaturally to the front of the bleacher, but they may help if you have back trouble.
Stinger Seats are nothing new. My father got them for us in early 70's. Things have changed though. Use to rent them per game back then. You'd have to pick them up then return them. Now season ticket holders can pay for use of them and they put them in the stands. They've been doing it the new way for years. It may look silly to some, but it's a good idea. Doesn't look silly come game time when there are people in the stands.