Java unreachable catch block compiler error

Why in Java can we catch an Exception even if it is not thrown, but we can't catch it's subclass (except for "unchecked" RuntimeExceptions and it subclasses). Example code:

class Test {
    public static void main(String[] args) {
        try {
            // do nothing
        } catch (Exception e) {
            // OK           
        }

        try {
            // do nothing
        } catch (IOException e) {
               // COMPILER ERROR: Unreachable catch block for IOException.
               //This exception is never thrown from the try statement body
        }       
    }
}

Any ideas?