This is because of category shuffling
iTunes and Apple TV UI (user interface) has a category tree similar to the one i use in my online store
Sometimes they recategorise a file and movie it to a new category
For example, i Purchased a Movie Bundle of james Bond movies.
Then the Bundle was reupdated with a new look
Going to this Bundle after that shows me a 'buy now' button, but i alread bought it
Going to view each individual movie allows play, but the Bundles never appear as 'purchased'
I also once got ripped for 40$ with the Lord of the Rings Trilogy - i purchsaed the extended edition
then a week later i could not find it in my library so went to 'redownload' it from iTunes
The bundle said 'buy now' and i thought it was just a login fail, and that if i purchased 'buy now' it would then after login tell me 'this is already purchased would you like to redowbnload'
But it didnt, it charged me another 40$
Later i found out that apple has about 3 different Lord of the Rings Trilogy versions, so now i have the standard trilogy and the extended trilogy, because of deceptive product display (search produced only one trilogy bundle, deceiving me into believing it was the one i had already purchased)
I have various items which are purchased but grayed out and wont allow download
One could also assume the possibility that it is deliberately or accidentally set to not permit offline viewing by the people who uploaded the file to iTunes for sale. Dont know if that is possible, if not then its Apple's Boob.