Given their rocky history with mobile GPUs, Nvidia was crazy to even consider overclocking.
Back around 2008, Nvidia made changes to their mobile GPU manufacturing process—particularly to the solder "bumps" that both bonded and electrically connected the GPU chips to computer circuit boards. The cumulative result of these changes was an astonishingly high failure rate in notebook computers. The technology press predictably hung the label "bumpgate" on the whole mess.
It would be another year before Nvidia would officially acknowledge that the problem even existed and another year before they would settle the class-action lawsuit for $200 million. Add the inevitable direct suits from computer manufacturers and years of consumer distrust, and it's easy to understand why Nvidia's stock value took such a severe hit (from which it still hasn't fully recovered).
Given Nvidia's troubled past with thermal stress and GPU failures, their choice to disable overclocking for mobile GPUs is not terribly surprising, nor especially controversial.