Seems pretty logical, really. In the practices and other scrimmages, you’re still talking about a more highly controlled situation than a spring game. In the spring game, you’re that much closer to game situations – particularly with 50,000+ fans on hand. The adrenalin level goes up, the effort goes up, the speed goes up – the chance of injury goes up. That’s particularly the case if the spring game permits the defense to blitz – all of a sudden, the line and backs are having to pick up players that they weren’t expecting. Since this is an evaluation time, the green shirt allows the coaches to evaluate how everyone is doing on the blitz packages while still preserving the quarterback’s life.
As for punt returns verses kickoffs, that one’s even more logical. On kickoffs, the blocking scheme is paramount – defenders filling their lanes and the offense ensuring that the wedge is set up properly. That’s missing on punts – once the punt is off, it’s more effort than execution. The blocking is not as structured or as organized – there’s less to evaluate; mostly, the effort. I would venture to guess that, statistically, there is a far higher probability of injury in punt situations as opposed to kickoffs.