In my mind, scroll bars in the background distract the user on the overall objective and on the main purpose of the light box. In the same light, most users hate popups and almost feel tempted to leave the website on the spot. Scroll bars in the background may tell users:
Yes, we made a mistake with this popup. However, we still have more content to offer. Scroll to find it...
Twitter disables scroll with you click "Log In." Team House allows you to scroll in the background and even scroll past the popup. Swagbucks allows you to scroll in the background, but keeps the popup centered in the middle (fixed positioning).
What is the most user friendly approach?