Decent runners score from 2nd on ground singles to right all the time. Polanco should have a good lead, is off on contact hit behind him. And it's not like the right fielder would be playing shallow on Cruz. That's just passive baseball. And with Kepler due up, against a LHer, there's a decent chance of a K, which is what happened. That run has to score, and there's no reason he shouldn't have, if he's running hard on contact.