I think the Nats have to know the ground-rules at Fillies Park...and Werth especially should know unless they have changed the rule about the railing since he play for them. I don't agree with FP: (a visiting team can't be expected to know all the ground rules.)
Teams get paid to do this.
I even remember one of the rules at old Griffith Stadium: the CF flag-pole was considered part of the fence. If a ball hit the flag-pole and dropped back onto the field, it was in play. Can't imagine that happened often, but that was the rule.
Werth should have known and should have run hard to score. LaRoche should have looked for an umpire to be giving the home-run sign. No sign = ball in play.