Not true, unfortunately. When you buy a movie or song from the iTunes Store, you buy the file. However, unlike a DVD, you can make 1,000,000 copies of your iTunes purchase for backups, and have it on up to 5 computers and any amount of connected Apple-enabled devices.
So, you buy a movie from the iTunes Store, you download it, and back it up to your external HD and your laptop. One of those things fries, you restore from backup. You can try getting a lost purchase back from Apple, but they'll only make so many exceptions, and they don't have everything on the iTunes Store forever, so your file might just be gone.