The RPO makes it difficult for lineman to get downfield, because well, you never know if the play is truly going to end up being a run or pass, as the QB has the option to hand it off or pass..hence "Run-Pass Option (RPO)". If he passes and the lineman get to the 2nd level and block the LBs, that is illegal man downfield.
Am I missing something with the other responses? Strategy seems pretty clear cut to me. Outside of Akron games, it does get called, though seems refs miss it plenty as well.