why do I have to pay for Amazon Prime Video when I have Amazon Prime?

You shouldn't be charged if you already have an Amazon account with which to log in.

If you choose to log in to the Amazon Prime app with your Apple ID, then its an entirely different account.

Log in with your Amazon account instead.

As I said, your iTunes account and the Amazon account with Prime are 2 different accounts. They have different features attached to them.

You may want to cancel the Amazon subscription through itunes, and log in with your Amazon account that has Prime to the Amazon services and apps on your Apple devices instead.

Amazon don't make things clear about their video library. They tend to describe all movies in their library as being Prime, but when you actually look at the films available you find that only a selection of them are 'Prime movies' and available to subscribers at no further cost: the majority of films still have to be paid for at the same rates as for non-subscribers (out of a claimed 'over 100,000' movies 'over 40,000' are included with the subcription). 'Free' (included) Prime movies are identified by a banner on the image:

If you are being asked to pay for a film which has this banner then it looks as if your sign-in hasn't 'taken' and you would need to take this up with Amazon.

Amazon is actually pretty good once you reach out and ask them for an explanation. I have been a Prime member since its inception, and while I agree in general with what Roger Wilmut1 said in his response, there is a large amount of material that is free if you are a Prime member. When in the video section, look for the Prime ribbon in the top left corner for items you can play without further charge. If the Prime ribbon is there, and you are still being prompted to pay, then that indicates you have not correctly signed into your Prime account. I would exit out completely, and then sign back into your Prime account. If your account is in good standing, you should see the prompts indicating you are in your Prime account. Hope this helps.

When you join Amazon Prime through their website (I don't know if it's different through Apple TV) you get free delivery on goods sourced by Amazon (as opposed to Amazon partners) and free access to those movies etc flagged as 'Prime' (about 40% of the total library in the case of movies) - items not flagged as Prime have to be paid for the same as for non-members.

You may find that the Prime selection contains enough items which appeal to you to justify the subscription.

You would have to contact Amazon regarding any billing issue.

Thanks but I just realized that I'm being billed by iTunes for Prime video on my Apple TV & not Amazon through with I do have prime. very confusing & still trying to figure out the monthly $8.99 membership fee>