Its about backward compatibility. Something like a fork of an official plug would be hard to maintain, and Scirra can't guarantee it as always usable.
For example there are countless third party plugs that are not maintained, and subsequently no longer work.
Also, many of the bare minimum plugs are like that because you can replicate them easily in events.
Scroll to for example is as easy as using the system scroll.x, and scroll.y.
The plugs simply offer a quick and dirty way to get something up and running, while keeping you from having to commit resources to something you're not 100% sure about.
That being said, an official git type repository for third party plugs would be a good idea.
It might make it easier to keep some of them up to date.