It has not been tested in court yet, however the theory is that safe harbor laws apply.
Safe harbor laws?
They are a part of the DMCA that protects internet service providers who transfer illegal data with an automatic protocol. I think the intended case was to protect ISP so they would not need to police their network.
If you do enable it, my presumption is that by demonstrating that you have tor running you can show that traffic from your connection is as likely to not be you as to be you - but I'm not sure if this is backed up by law, or has been tested in court.