from everything that I've heard - The Forest Preserve District of Cook County (FPDCC) has the active jurisdiction over that area and they are extremely strict because of all of the vandalism that has occured over the years. The normal hours of operation for Bachelor's Grove Cemetery are the same as they are for the entire Rubio Woods Forest Preserve area....dawn to dusk, strictly enforced! If you are caught there after dark You will be arrested for criminal trespassing!
So you can be in the cemetery during the day - it's open to the public, but after dark, you are tresspassing if you go there. I have even heard about a local radio station that was going to do a "remote broadcast" from Bachelor's Grove at night (I think it was one year on Halloween) and when the DJ and technician showed up - they were arrested.