Virgin's move was probably an effort to distinguish itself as a higher-end prepaid service. Along with competition from other MVNOs, Virgin has its cousin Boost to contend with for market share. Striking this deal with Apple gives it visibility in all their stores as well.
Sprint already severely restricts which devices can go on their networks by whitelisting devices, having various activation rules involving their pre vs. postpaid brands and the overall difference of Sprint's network from their competitors' which makes their devices somewhat unique. They should be aiming to relax these rules as much as possible, not restrict them, moreover so in prepaid brands in order to gain as many customers as they can. It's amazing how many bad decisions after bad decisions this company keeps making, regardless of who's at the helm.