The ugly fact is the Unlimited Amazon Drive has been abused by media pirates and data-hoarders to store up to 20TB (and sometimes more) at what any reasonable person would say is an unreasonable cost. Not to mention, much of this media is accessed and streamed often. Wander over to /r/PlexShares to get an idea: imagine if someone had 20TB of 4K and BluRay movies stored on your service @ a very generous $60/yr, streaming that data 24/7 to 10, 15, maybe 20 people via Plex (with each streamer paying the media manager $10 a month) all over the world. While torrenting all day. Suddenly that sounds more like abuse IMO.
Of course, I agree that you shouldn't call something "Unlimited" if it isn't. But it's not a one-sided issue and I thought I'd bring that up. I don't personally know anyone, in real life, who uses Amazon Drive. Most people don't even know it exists. The only time I see it discussed, especially the "unlimited" tier, is on /r/datahoarders and /r/seedboxes as an exploitable deal.
As far as I can tell, Prime Photos will remain unlimited. I wonder how long it'll be before media hoarders hide their content in Google/Prime Photos with a convenient CLI tool?