Why do I need to be on Wi-Fi to watch downloaded movie on iPhone?

I downloaded the hunger games on my iPhone 4s and it refuses to let me watch it unless I have Wi-Fi! Why does it need Wi-Fi for an already downloaded movie!

Added (1). Thank you! The Wi-Fi never works in my room but when I wake up tomorrow I'll play it all the way through. I was going to be pissed if I needed Wi-Fi everytime. For some reason whenever I have Wi-Fi on it stops my Internet and phone connection from working. Anyhow, thanks again!

Same thing happens to me, what you have to do is let it play at your home or Wi-Fi all the way through and then play it on the road or wherever you are without Wi-Fi. The reason is because it's still got gigabytes and things to load, so once it plays all the way through on a Wi-Fi network (you don't necessarily have to watch it or be by it: maybe play it while your sleeping and turn the volume off and leave it charged) you'll be good to go and play it wherever! Hope I helped!