It's not a feature, it's a bug: Nvidia claims that laptop graphics "were not designed" for overclocking and that it's "safeguarding" users' systems rather than taking features away from them.
http://www.notebookcheck.net/Nvidia-disables-overclocking-for-900M-series-graphics-cards-with-a-driver-update.136576.0.html
Just correcting you.
Maxwell 860M and 950M are in the OC lock list too.
And the first driver with lock is 347.09, not 344-series.
Yesterday official news from NVidia is that they have listened to their customers and will be re-enabling overclocking on their GPUs with their next driver release, see following link:
https://forums.geforce.com/default/topic/805791/geforce-drivers/gtx-900m-overclocking-with-347-09-347-25/post/4466443/#4466443
This is great news, NVidia have listened to us!