Again, I'll restate I'm not an aerodynamics expert, just a mechanical engineer thumbing through his old turbomachinery books...

The airfoil shape doesn't have to change from root to tip...in fact, I would argue that many models don't. Since the airfoil thickness is specified as a percentage of chord, scaling down the tip airfoil uniformly will yield the same relative thickness, albeit thinner dimensionally. For example, NACA 0014 is 14% as thick as the chord, whether the chord is 10 inches or 10 feet.

Nonetheless, it's fairly trivial design-wise to change the section from root to tip...most of the airfoil packages I've played with do it. I imagine it is done mostly to alter the wing tip behavior (increase or decrease stall tendencies, for example as Mike suggested), but it might also be done to increase the wing strength for really thin wings for racing planes.

For what it's worth, in full scale aerobatic planes, some have the same thickness (by percentage, not physical dimension) and many are actually thinner (Extras, Sukhois and Yaks, for example). Didn't see any that were thicker at the tip in a cursory review. An interesting link of full scale aerofoils for those who are interested:

http://amber.aae.uiuc.edu/~m-selig/ads/aircraft.html