A spokesman for the Association of Convenience Stores said: "With something like this, what can you do?"
He added that external CCTV equipment did little to stop someone who was intent on this sort of crime.
The latest attack occurred on April 27 at a Pricecutter store in Kennington. Scotland Yard has linked the event to firebombings in Tulse Hill and South Norwood. Asian shopkeepers were targeted in all three attacks, raising the spectre that they were racially motivated.
The Grocer Top 50 Scottish retailer CJ Lang added: "This situation would be improved if the press ignored the actions of a few idiots."
Kishor Patel, who manages a Nisa Local store in Luton, Bedfordshire, said: "These attacks are more isolated than a few years ago, but retailers should be vigilant."