I don't know for sure, but I imagine that line of tall trees was planted specifically to hide the KH from those visiting the historical site and vice versa. The JW's wouldn't want to be visually associated with even a historical site related to slavery.
Ironic, considering that their precious Bible doesn't condemn slavery at all. It fact, it presents it as a societal norm in both the Hebrew Scriptures (Old Testament) and the Greek Scriptures (New Testament). What scant few regulations regarding slavery there are basically protect the slave owner or prohibit only the most egregious mistreatment of slaves. In the New Testament, slaves are ordered to be obedient to their masters. Not even a hint can be found that slavery is wrong in and of itself. Thank goodness society as a whole has outgrown the Bible in that regard. Now if we could just get rid of all the other superstitions...