how does whatsapp service gets restarted even if i force stop app?

Before you up/down-vote this answer please take into account that the answer is nearly 3 years old now (speaking of May 2015) and things may have changed!

I don't think it's some magic what happens here! It's just Android C2DM (see: https://developers.google.com/android/c2dm/), whereas the app has a registered Receiver for incoming Push Notifications and gets awaken by this message.Android C2DM is/was available with Android 2.2, that's the reason why you can't see the same behaviour on your device with Android 2.1 up and running.

By the way: As you can see, C2DM is deprecated since June 26th, 2012. So instead of C2DM, one should use GCM (see: http://developer.android.com/guide/google/gcm/gs.html)

Useful Comment: GCM needs available internet connection. You can using any other broadcast receiver such as SMSReceiver for by passing this limitation.

Starting from Android 3.1 (API 12), if an application is force-stopped it will not restart until the user manually runs the app again.

This will happen even if the app contains a Service or an active BroadcastReceiver.

You can find the official documentation here.

i dont have idea about whatsApp service.

But it is possible that after force stop application, restart service of app.

i use START_STICKY service for my chatApp. i have to do same thing so i use START_STICKY service so when my app kill or force stop from setting, after few second my service get restart and i able to login to my xmpp server and get incoming message.