It seems doing so anonymously might be a bit tricky for folks without technical knowledge. Hell, I'm a programmer and I still would be afraid to publish without the guarantee of anonymity provided by a "trusted" third party.
They also provide protection for people who they deemed to have aligned interests - e.g. Snowden.
I agree though that there shouldn't be a filter on what gets leaked. But it may be necessary given the fact that the US government has the resources and laws that it does.
