if itunes on your computer don't let you then likely apple don't have rights to it in your country
if itunes on your computer let you then maybe the country settings in your appletv (set in the appletv menus) are set wrong
I can purchase movies on iTunes on my computer I used to be able to purchase movies via Apple TV all the settings you have mentioned seemed to be okay here also. I have purchase over 130 some from iPhone. PC. And loads from Apple TV. Strange one.
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