OK, I have just tried this with an app which I haven't rated before and I know it used Rate-My-App to bug me to rate it.
I selected 'Rate with 5 stars', the Store opened, and the 5 stars were NOT preselected.
I've also looked at the source code for the Rate-My-App quickly and found nothing that's setting the stars value.
My guess is that you already reviewed the app with 5 stars before, so when you opened the review task, it preselected the number of stars which was already submitted before, giving you the impression that it was predefined from inside the app.