IIRC, Rick and Shane are actually from Kentucky and served a Kentucky police force, so might not know that there’s a prison near Herschel’s farm, just to give the two cops the benefit of the doubt. Besides, by the end of the finale, Shane’s dead anyway, so he’s not around to remind the group that, hey, guess what, guys? A prison!

OTOH, I am surprised that Herschel and his family, as long-time residents of the area, don’t know about the prison…