It wasn't so much a policy decision, it was more a question of picking battles.
FERMI GPUs are very complex, with elaborate clock trees and memory interfaces (e.g. GDDR5, EDC, ECC, etc.). The NV-CONTROL overclocking interface, on the other hand, is quite naive. In order to properly support clock manipulation on FERMI and newer GPUs, a fair amount of work will need to be done in at least the X driver, NV-CONTROL and nvidia-settings to overhaul the overclocking infrastructure. I believe on Windows, a lot of this type of functionality was moved outside of the driver for that reason.
We do have a bug tracking this RFE internally, and I expect we'll get to it eventually. But given ever-increasing hardware/software complexity, general driver quality concerns and other long-standing feature requests (such as the long-neglected RandR extension), it's hard to justify the effort for a small subset of the NVIDIA Linux graphics driver user base at this time.