Those in control of Jo hadn't been counting on an extra person in the forest. In fact, they didn't even see Mary-who was obscured from view in the vantage point that Jo was crouched at.
Instead they took aim and waited for the elder Winchester to hit the ground. They might have heard a few noises as Mary made her escape, but it could have simply been an animal and a small search of the area hadn't turned up much.
Time was wasting, so Jo's strength was used to drag the intended sacrifices to the gate and bind them. Then the ritual to trap the reaper was started. It would take awhile. Long enough for the Winchester brothers to perhaps return to consciousness. And possibly long enough for Mary to figure out how to handle the situation at hand.