Why does `instanceof` error rather than return `false` when used for 2 incompatible classes?

I'm reading this:
http://java.sun.com/docs/books/jls/third_edition/html/expressions.html#15.20.2

They say:

Consider the example program:

class Point { int x, y; }
class Element { int atomicNumber; }
class Test {
        public static void main(String[] args) {
                Point p = new Point();
                Element e = new Element();
                if (e instanceof Point) {       // compile-time error
                        System.out.println("I get your point!");
                        p = (Point)e;           // compile-time error
                }
        }
}

The instanceof expression is incorrect because no instance of Element or any of its possible subclasses (none are shown here) could possibly be an instance of any subclass of Point.

Why does this result in an error, rather than simply in instanceof returning false?

Thanks,

JDelage