Let's be thankful that for most of us, this falls under the "no duh," category. Handicap parking spots are designed with loading and unloading zones on either side of them, so that vans will have room for their ramps to raise and lower for wheelchair accessibility. If a motorcycle is parked in this spot, or even worse, if there are motorcycles parked in ALL of the spots, there is no way for a handicap driver to exit the car in his or her wheelchair.
If you see someone doing this, let's just assume that they don't know what those zones are for and try to politely inform them of how their actions affect those have a harder time getting around.