I'm not a fan of automatic donation either: the main reason why I disable it myself is that I prefer distributing my donations to developers manually to better keep track of my donations (I have donated to ckolivas, lukejr and kano too).
But I understand why it makes sense for forrestv, most users wouldn't donate anything otherwise.

Currently (https://github.com/forrestv/p2pool/graphs/contributors), forrestv is by far the largest contributor, with more than 75% of the commits (not counting merges for code or content contributed by other developers which isn't free).
So it's not a problem currently: no other developer can claim having contributed meaningfully without looking like a troll. The second contributor way behind forrestv with ~1% of the commits is coblee who provided the litecoin support (which served as a base for other scrypt support). I'm not sure who gets the donations when using p2pool for litecoin, it might be coblee.
If someone else took over or even contributed large enough amounts of code they would have to ask forrestv if they can receive part of the donations or fork if they can't agree on this.