Why Thread.sleep() in main thread causes children threads to not shutdown

Question

In the example below, I have main program that creates 6 tasks and 6 threads (1 per task). As you can see, the code is really very simple. I create and start 6 threads and then I want to stop all the threads through a shared variable/flag shutdownIssued.

However, I am seeing that the main program and threads don't ever terminate (they keep running and I have to forcefully terminate them). But the program works as expected if I comment out first Thread.sleep(1000) (ie., before calling shutdown() on all the threads).

1: Why calling Thread.sleep() on current(main) thread doesn't cause threads to stop (which means that shared variable shutdownIssued inside the threads is not really updated by a call to shutdown() by the main thread).

2: Why does this work as expected when I comment out first Thread.sleep(1000)?

Example code:

package com.examples.threads;

import java.util.ArrayList;
import java.util.List;

public class ThreadExample {
    public static void main(String[] args) {

        //create 6 tasks and 6 threads to run them
        int numTasks = 6;
        List<Task> tasks = new ArrayList<Task>();
        List<Thread> threads = new ArrayList<Thread>();
        for (int i = 0; i < numTasks; i++) {
            tasks.add(new Task());
            Thread th = new Thread(tasks.get(i));
            threads.add(th);
            th.start();
        }

        //Wait for 2 seconds before issuing shutdown
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

        //Now issue shutdown request to all the threads to stop execution of their assigned task
        for (int i = 0; i < numTasks; i++) {
            tasks.get(i).shutdown();
        }

        //Wait for 2 seconds after issuing shutdown
        try {
            Thread.sleep(2000);
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

        //Check if threads are still alive
        for (int i = 0; i < numTasks; i++) {
            System.out.println("Thread " + threads.get(i).getName() + " is alive = " + threads.get(i).isAlive());
        }

        System.out.println("finished shutdown of all the threads!!");
    }

    public static class Task implements Runnable {
        private boolean shutdownIssued = false;
        private String name;

        public void shutdown() {
            shutdownIssued = true;
            System.out.println("Issued shutdown to " + name);
        }

        public String getName() {
            return name;
        }

        public void run() {
            this.name = Thread.currentThread().getName();
            System.out.println("started thread " + name);
            int cnt = 0;
            while (!shutdownIssued) {
                //System.out.println("inside thread " + Thread.currentThread().getName());
                cnt++;
            }
            System.out.println("Finished thread " + name + ", count = " + cnt);
        }

        public boolean isShutdownIssues() {
            return shutdownIssued;
        }
    }

}