If you make an error such as forgetting to set the computer to the correct mixture of Nitrox, the computer should allow an in-water correction otherwise a computer is reflecting an incorrect state of gas loading. The programmers could write routines which allow for this correction, but obviously they don't trust people not to give themselves more BT than deserved. There is no other reason that can account for not allowing this correction. Why doesn't a computer allow you to switch it to gauge mode after diving a dive in computer mode? Again, it doesn't trust you to make a good decision.
You pretty much put the computer in control of your diving. I can imput reality into a software program like GAP, Vplanner or DecoPlanner and get the real picture, but not an in-water computer like a Suunto. I use my gas switching Tusa computer more often as a computer than the Suunto Cobra because I can program 2 gasses and switch in the water.