True, there are security issues, as you noted in your Holmes quotation, but when a person with a pro-Bush sign is allowed up close while a person with an anti-Bush sign is relegated far away, it ceases to be a security issue.
It is more likely that a person with such a sign poses a clear and present danger of imminent lawlessness . It is a judgment call. It is narrowly tailored, and there are sufficient avenues of expression open to such a person.