To say that it's impossible to sneeze with your eyes open isn't entirely correct. The process that produces a sneeze affects many parts of the body, especially the muscles of the face and abdomen. Part of that sneezing process affects the muscles that control the eyelids, and your eyes close as a reflex when you sneeze.
You can, however, fight this reflex in much the same way that you can fight the gag reflex when you try escargot for the first time: If you know what's coming, you can prepare yourself for it. Certainly, if you feel a sneeze coming, you can physically force your eyes to stay open when you sneeze (and no, your eyes won't pop out), and people have said that they've been able to train themselves to keep their eyes open, but both of these involve going against what the body does naturally.
As for sneezing in the car, the time it takes to sneeze (and therefore the time that your eyes are closed) is very short, certainly not any longer than the time it takes to blink or to look in the rearview mirror or to check the speedometer — and none of those things are illegal.
The worrisome part about sneezing while driving isn't that your eyes close, but that when you sneeze, many of your muscles suddenly tense and then relax, including your arm muscles, which could jerk the steering wheel in a surprising way. Be extra careful if you feel a sneeze coming on while you're turning.