It is a license that touts "ultimate freedom" but the reality is that it removes your freedom to make changes that you would like to keep private.
The basic idea is to avoid someone taking something like a web server, heavily build on it, and set up a big service running it -- but not providing the modifications back to the users of the service (and eg: leaving them "out in the cold" if the service shuts down). Or in this case -- if you fork pinry, that's fine -- but your users are entitled to the source and your modifications (note your users not everyone).
I think it's a perfectly valid licence, and appropriate for stuff like this, where you could take pinry, set it up as a service, slap some ads on it, and make money off of the value created by pinry's authors. Nothing wrong with that. But if you find and fix bugs, those fixes have to be contributed back to at least your users (which then again are free to redistribute them under the AGPL, making it likely that they find their way back to the original project).
A regular GPL with a constraint on number-of-users seems fine for protecting the openness of a product intended for personal (non-hosting-provider) use, and pinry can license a commercial version for hosting providers.