Those are meant as indicators for which end is minus and which is plus. When you click, it sets the slider position nearest to where you clicked, so you are getting minimum or maximum zoom!
You can mouse over the timeline and use the mouse scrollwheel. I've made a note so we allow the scrollwheel on the zoom slider.
In the graph you can hold alt and right mouse drag to scale the graph. We plan to add the same to the dopesheet.
