The sticking point with the Model S, and other vehicles like the Chevy Impala and Ford Taurus, is down to a specific examination the IIHS runs. Back in 2012, the body began running the Small Overlap Frontal Test, which crashes into 25 percent of the body at 40 miles per hour. The examination is the equivalent of some yahoo crossing the median strip on a highway and smashing into your front fender. It's the sort of crash that puts far more stress on your car's structure, and also causes plenty of serious injuries.
The agency takes great pains to stress that the Model S is by no means an unsafe car, earning second, rather than first, place in the test. The reason for the demerit is twofold: the safety belt was too loose, and the crunch encroached on the cabin too much in one of the tests. The IIHS also noted that the Model S' headlights aren't the best, but that it also has yet to test the higher-performance versions of that same car. So, in summary, the IIHS says that the Model S is pretty damn safe, but there are circumstances where it may not be the safest.
Of course, Tesla has a -- somewhat justified -- persecution complex when it comes to negative stories from the automotive industry. As a new entrant with a status-quo defying power train, it serves as a threat to the century-old establishment. It took the BBC's Top Gear to court for saying its cars frequently ran out of charge, although it lost that defamation suit. Two years later, and the company had a public fight with The New York Times, repudiating a negative review by publishing the test vehicle's telematics.
Tesla has already responded to the claim, pointing out that the Model S still has an unimpeachable record at the NHTSA. Elon Musk's team also makes a snide reference to industry groups having their own "methods and motivations that suit their own subjective purposes." It's worth saying, too, that this isn't the first non-Governmental body to demerit Tesla on safety -- Consumer Reports did similar back in April. It looks like Tesla may have to set up a new safety commission, as well as ones to tackle its other internal problems.