Unprivileged users are users who have:

These users are mostly hosted by free web hosting services, most likely associated with their ISP.

If you change perspective on this list, and look at it as prerequisites to levels of Atom usage, it looks something like this:

To host an Atom feed and Atom-format entries, your ISP and/or publishing software must provide:

To host a publishing system that supports the Atom API, your ISP and/or publishing software must provide:

A related survey of incomplete implementations of HTTP can be found at CarrotVsOrange, which lead to DifferentlyAbledClients, which in turn was the motivation for the [WWW]SOAP Enabling section of the draft-gregorio-09 version of the AtomAPI. PaceWsdl continues this work; PaceAtomActionHeader has been forward as an alternative.