Because the particles are not soluble in gasoline, putting anything grainy like salt or sugar is bad for the fuel tank, various filtration systems and fuel pumps. Adding salt clogs and corrodes these parts, requiring intensive cleaning and replacement.
A car with salt or sugar added to the gas tank requires a deep cleaning of the fuel tank and replacement of the fuel pump and filtration system.
People have tried similar tricks with sand or golf balls. The sand is not as damaging because it is heavier than salt or sugar and sinks to the bottom of the fuel tank, but golf balls are very damaging because the fuel tank causes the balls to unravel.