True, with accordion and other hierarchical controls, the right/down arrows are a state indicator and a clickable item. If the arrow icon was only a button there might be a problem, but as a part of an expandable grouping and display control it's such a common convention that most of us don't even think about it. jQueryUI, Bootstrap and other frameworks use arrows in this way.
Sure, +/- icons on accordions actually follow the "button should indicate action" rule, but I prefer the look of arrows as they seem more clearly descriptive to me.