Apparently she didn't have a permit so they took her signs:
The requirement of a permit is sometimes denounced as an infringement of free speech, as permits are denied on spurious grounds or protestors are corralled into free speech zones. Permits are sometimes denied on grounds that the protest will create a security risk. There seems to be evidence that the available venues for protests are shrinking in number; that citizens have experienced increasing difficulty in gaining unrestricted access to them; and that such venues are no longer where most people typically congregate in large numbers. In Washington, DC, the National Park Service Police, U.S. Capitol Police, and Metropolitan Police of the District of Columbia have an elaborate permitting system. Many famous people like Martin Luther King, Jr. have been arrested for protesting without a permit.