Empty page.tpl.php renders data too
This is the expected behavior if page.tpl is missing:

Since your theme does not print anything in page, the Core system module (the one that defines page.tpl.php etc) takes over and drupal uses system's page.tpl.

I suppose that if page.tpl is empty the default page array is rendered with any region that exists in your theme's.info and has content.If regions exist, Drupal will try to render them even without template.