They were publically accessible (as in no authentication was required) regardless of how you want to spin it. But if that's not good enough for you...

It's my understanding that megaupload (and it's other sites) links were of the form: http://www.megaupload.com/?d=XXXXXXXX

I could be wrong, but the links I've found so far all have 8 characters.

Notice it's a somewhat easily searchable space. You can enumerate all links starting from point1 to point2 (I think they assigned sequential numbers so no need to search the entire space).

Hence the slew of public MegaUpload-search websites that you could go to and search for every file in the index.

The point of the response was that if you needed private backups of your Amazon MP3 you should use a service that is a true lockerbox, and not a pseudo sharing site thats designed for sharing content. Otherwise, you need to take the blame for 1) losing access to your backups, and 2) knowingly sharing it even if you did not give the link out.

A lockerbox service that requires some authentication, or a very large GUID type link that can't be enumerated.

MegaUpload was designed for sharing.

The case of someone losing their legitimate Amazon MP3 upload is an edge-case that inconveniences one person, but takes down the sharing of hundreds, or thousands, of copies of illegitimate copies.