Feel like 15 is a bit much considering the QB can directly listen to the coach. I get in the old days that penalty could be needed because technically someone could listen to the coach and then run in, let everyone know, and then run out...
But these days? With the green helmets or whatever.. seems excessive. Especially when it appears it's just a mixup. You obviously don't want guys running out there when they're not supposed to. Could get delay of game, mixup/12 man on field.