There is a rule in training camp that you aren't supposed to tackle the quarterback. The red shirt reminds them of it. That is it. However, the red shirt will not prevent the defense from pushing the linemen/TE/RB's from getting pushed into the quarterback. When this happens it is called an accident.
Contact sport people stevestojan happens.
I feel bad for JP and I liked to watch him play but in the end he will learn more from it.