Below picture is what the hole looks like (picture ganked off the web somewhere). 10 feet below that guy is the dead cow.
The problem with putting a heavy gate there is that most people won't be entering this hole, but trying to climb up out of it, having entered the cave at the big entrance and done a whole through-trip.
It's a slippery vertical shaft and having to lift a gate while climbing it would stop most people if they didn't expect it to be there, and didn't go move it before-hand. You'd have some falls and maybe injuries there from some mud-covered fellah who didn't expect the gate and just travelled the whole cave from the other side, expecting a few easily-moved sticks at the end.
I can't imagine the landowner tossing the cow down there, knowing the amount of summer-camps and youth groups that have used those two caves (Gandy and Stillhouse) as conservation classrooms for untold years, (unless someone really made him mad). Kids go through there all the time, and he's surely seen and given permission to bus-loads of them over the years.