Please also read and preferably discuss in https://devtalk.blender.org/t/particle-nodes-ui/8808/.
I actually agree that it is less intuitive to have the actions plugged into the influences from the left. In fact, my first proposals worked like that (they are linked from the first post in the linked thread). Even more, I actually argued that it should work like that quite strongly (see https://devtalk.blender.org/t/particle-nodes-ui/8808/71).
Unfortunately, there were serious issues with this approach that are explained in the second proposal.
Since then I was looking for a solution that solves these issues. The current solution came from user feedback as well as from looking at how this is solved in Softimage ICE. After some thinking I decided to use the solution I’m using now (having the action nodes in the left side of influences). That really was not an easy decision, because I used to think the same way you are thinking now.
It was a trade off. In the end I valued more powerful node groups and a more structured node tree higher than the initial intuitiveness of the system.
Fortunately, after working with the system a bit, the mind adapts quite quickly to this approach.
@lsscpp This could be done relatively easily, but I’m not sure if it should be done. For now I think it should just be a single input. I actually like that this is the only socket that allows multiple inputs because 1) it signals that this socket type is very different from the others (and it is!) and 2) the influences do not have a natural ordering. I prefer not to present an order to the user if there is none.
Fortunately, if I change my mind, I can update it quite easily later on.