They honor people who died in a war.  Most people on the left consider the Iraq war an unjust war.  Do you disparage memorials to Iraq war casualties?  The vast majority of southern soldiers weren't fighting over slavery.  Their political leaders may have been, but the soldiers were fighting for each other and what they perceived as a right to govern themselves.  The issue of federalism and state's rights was a fundamental issue and pre-dates our founding.  Is the left now blaming soldiers for the wars their leaders get them into?