Peterson is still going to be returning punts for the Cards, they just brought in Ginn to ease up on the amount of times he has to do it since Peterson is also playing on the offence.
And the Cowboys are still letting Dez return punts in practice just incase they ever need him too do it, so they have no hesitation putting him back there. http://cowboysblog.dallasnews.com/2014/06/dez-bryant-returns-punts-at-minicamp-jason-garrett-hell-be-in-the-bullpen-if-we-need-him.html/