Originally Posted by JD Gumby
DRM isn't just encryption - it's anything that is meant to enforce the will of the provider of the content when it comes to access or distribution.
That is true.
In general, though, Paywalls are considered more along the lines of access control since, once you have authorized access to the content you can do as you wish with it; copy, save, even download. So they don't really control distribution.
But yes, access control is a DRM feature and a Paywall is a form of DRM.
So are undocumented formats that are only usable one a single app, when you get down to it. (iSilo comes to mind.)