Why only medicine lectures are closed to the public?
In the United States, the Health Insurance Portability and Accountability Act (HIPAA) greatly restricts who can see or discuss medical records. While most grand rounds tend to be obscure the patient's identity, there could be an abundance of caution. Note that there is an exemption from HIPAA for grand round presentations -- if the audience are only medical personnel (including students):

The HIPAA Privacy Rule allows physicians and staff to use and disclose PHI [private health information] without a patient's written authorization for purposes related to treatment, payment, and health care operations.  It further defines &quot;heath care operations&quot; to include &quot;to conduct training programs in which students, trainees, or practitioners in areas of health care learn under supervision to practice or improve their skills as health care providers. [Cite]&quot;



The other reason in the United States that grand rounds are often closed is the prevalence of various types of protesters -- e.g. from PETA or from the anti-psychiatry movement.

[That all being said, at my university most of the grand rounds are open unless closed at the request of the speaker or for other reasons. However, they aren't advertised so you would have to be a department member or on the mailing list to know when and where to go. They are also behind the security umbrella of the university, so you would need an ID card to get past the guard at the front door of the building. So whether you call that 'open' is up to you.]