The onClick property is for client side JS, and not SSJS. This means that your SSJS is executed when the page is rendered, and then the generated client attribute is the result if your formula, like sessionScope.put("selectedCenter","Ft. Pierce")
To execute SSJS on the onclick event, you should do 2 things:
1- Set the submit value of the label node to a unique value
2- Add on onclick event handler to the application layout object, and add your SSJS there. In your code, you can then check the submitted value to know which node was clicked
There are a few examples of this in the demo DB
Again, this is because the nodes are not components and can't have an server event handler assigned to them. Also, the nodes can be generated dynamically, like from a Domino view