"whether today's Confederacy enthusiasts agree or not, the Confederate flag surely sends that message to many of those who see it being flows -- particularly, of course, African Americans."
If today's confederacy enthusiasts disagree, then it means the culture doesn't represent slavery. Many in the south had relatives that fought and died for the confederacy. It means something to them, but this doesn't seem to matter to anyone.
"surely does not exist for the express purpose of cultivating terrorism"
The confederate flag doesn't exist for the sole purpose of slavery. Just like Islam representing terrorism, it's just one of the many facets. I would also like to point out that Islam does represent the subjugation of women (even if the the people part of the religion don't see it this way). Any woman involved in the religion has to keep their head covered (some, their entire body) and in pretty much all Islamic countries, the laws are heavily biased toward men. Homosexuality is also illegal and can mean jail time.
"And, more to the point, the public reaction simply reflects, I think, widespread agreement that this is the case."
A small, vocal, minority of people agree (like in most things). We have had the confederate flag around for many, many years. I don't ever remember even hearing a peep about people even giving a shit that it was for sale or flown. Not one person in government trying to get it removed or one article online (before this incident)
wanting it removed.
If it was really a huge issue, why the outcry now? And why are we blaming an entire culture for the actions of one person?