Why is flutter a function of true, not indicated, airspeed?

I am learning about gliders in preparation for an exam. It says in the textbook I am reading that for a glider, you use indicated airspeed for most relevant speeds (stall speed $V_s$, best glide speed $V_{BG}$ etc). However, $V_{NE}$ should never be exceeded in true airspeed. As far as I can tell, this is because flutter depends on true airspeed rather than indicated airspeed. (Let me know if this assumption is wrong, because that would make the title wrong.)

Why is this the case? Is flutter somehow related to the speed of the air molecules, rather than to the energy provided by them?

• Peter, I would like to make a strong recommendation that you change the question title to: "Why is flutter a function of true airspeed?" because you're already correct, at least partially, that it is because of flutter. – Ryan Mortensen Jul 20 '16 at 5:28
• @RyanMortensen good point. I will modify both the question and add a comment explaining. – Peter Jul 20 '16 at 5:31