Diode protection in Pins
"Radiation" is scary! ;-)

Rad-hard chips are designed for use in space and other extreme environments where exposure to very high energy particles is possible. On Earth, the atmosphere absorbs virtually all of these particles (which is partly why life on the surface of Earth is possible).

There are really just two concerns here:


Hardened for use in severely radiative environments ("Rad-hard") means that the chip cannot assume the presence of useful things like the atmosphere to absorb high energy particles. 
A floating (otherwise disconnected) pin, as is typical of DNC or NC pins, is a pretty good (small) capacitor. 


If incident high energy particles succeed in transferring energy to the floating pin the resulting stored voltage could be extremely large and this in turn could exceed the ability of the chip package to keep the charges isolated on the pin. It will break down and "zap" the sensitive micro-chip inside. 

To prevent this, shunting diodes are added to disperse the charges before they can build up to a level sufficient to tunnel through the insulation of the chip's packaging. 

But why not do this for all pins?

This is not necessary in all but the most extreme conditions as even cheap plastic packages and tiny sizes can resist thousands of volts. Pins that are not connected internally can store charges and then discharge them when touched or in contact with another body (e.g. ESD), but so what? If the package can prevent the discharges reaching the internal micro-chip and the external pin is not connected to anything (or grounded through PCB connections), there is no real consequence to it.

ESD diodes require a relatively large amount of area on the chip surface and the cost of a microchip, to the first order, is proportionate to its size. So, if you don't need to use a pin, there is little reason to waste space (increase the cost) to protect that pin.