It may not have started out that way (but it may have, I'm not sure), but MacPorts is an Apple hosted project: http://www.macosforge.org/
What they don't host is the source or compiled versions of any of the packages in the MacPorts repository - potentially for the same reason they include no GPLv3 software in their OS.
In some cases, they do. This is especially apparent if you're still using MacPort with PPC, as most of the packages it pulls down are form the MacPorts servers.
What they don't host is the source or compiled versions of any of the packages in the MacPorts repository - potentially for the same reason they include no GPLv3 software in their OS.