So presumably the problem here is that DRM in the video is causing it to behave like a subscription rather than a purchase?

It sounds to me like you still only rented it (but I've never used amazon video stuff - I did get a movie from apple/iTunes once and that was very clear on the time -based limitations).

I searched amazon video and its quite clearly pitching itself as another shitty subscription service.

I think once again this comes down to false expectations (and possibly shitty marketing?)