Ted Kinney, Director of Education for the Alabama Independent Agents association, graciously emailed me with a correction:
"Ferguson is probably more of a “civil commotion” than a riot. Most standard policies do cover riot & civil commotion. If a covered peril damages property on the policy’s described premises, the business interruption coverage (if there is coverage) would also be triggered."
He went on the explain that "riot and civil commotion are not defined in commercial property insurance policies and forms (or homeowners forms, for that matter). Instead, major court decisions ... provide the guidelines, often based on dictionary definitions or statutes."
So there you have it: the damages, and the concomitant loss of business, are most likely covered - assuming, of course, that the store owners actually had policies in force.