A commercial auto insurance policy covers vehicles used for commercial purposes. If a company's commercial uses of a vehicle include transporting items that the insurer deems risky (based on their actuarial analysis), then the insurer is free to decide not to offer coverage to that company. Many insurers do not offer commercial coverage to companies that transport hazardous items, and many define "hazardous" to include firearms and/or ammunition. Just like many companies won't offer coverage to companies that transport hazardous materials.
GEICO is, apparently, one of those companies - its commercial insurance coverage is aimed at white-collar businesses (fleet vehicles, etc.) and very light industry. It is entirely plausible, then, that a firearms manufacturer simply is not within their desired risk protocol, and thus does not qualify under their underwriting guidelines.
Are you suggesting GEICO will not insure commercial accounts involving hazardous material, explosives, fuel, bio-hazard, military hardware, etc, etc?
It is entirely possible that what you said is true regarding GEICO. However, given the date on the letter (1/22/2013), I’m betting that it’s just another PC spasm in reaction to Sandy Hook.