Because not testing it can have unintended side effects at some low level of probability, and we all know this - mostly we just don't think it's going to happen to us. However, as the significance of the risk associated with that low level of probability goes up the demands for securing against that risk go up.

Computers are very powerful when placed in certain configurations. The more powerful the system you're dealing with the more cautious you should be. If they were dealing with an app then, sure, I'd have a lot more pity for them not taking precautions - such precautions would not be reasonable to expect of them. But if you're not being excessively paranoid about such a powerful system as was deployed here, then you're doing it wrong.

I do feel some pity for them based on the fact that there's not a tradition of caution in programming. And I do agree that there were multiple points of failure in there. But testing all the code that's going to be on a system like this is a base level of caution that should be used - whether or not you intend to use that code. If you think it's too much bother to test, then it shouldn't be there - but if it's gonna be there then for god's sake test.