I think you're wrong to ignore the consequences of the actions as an input to the punishment. A small amount of unintentional carelessness that causes huge damage could still be punished. One could argue that a certain degree of mindfulness in critical situations is a job requirement, and casually making a careless -- though unintended -- mistake demonstrates a lack of mindfulness indicating that the person is not properly qualified for the job.
I understand that we don't want a culture that fires people for making the sort of mistake anyone might make. But to be so careless on a day that is clearly an exception where something more important than standard business procedure is going on, can't you at least see why firing the intern for such a lack of mindfulness might at least make sense, even if you disagree with it?
I interned for a government organization that maintains hydroelectric dams and the software that controls them throughout the Southeastern US. A careless mistake could -- in the worst case -- cause blackouts, cost the company millions of dollars, or even cost lives (if the data-control feedback loop caused a turbine to spin up at the wrong time or to fail to shut off in an emergency). And, as is quite common in organizations with non-software-engineers running the show, the development processes were entirely haphazard. The environment was such that it would be really easy for me to push unreviewed code, or to make a stupid deployment mistake, or to be careless in a number of ways that the system didn't protect me against.
But it was OK, because they hired smart, competent people who understand the need to triple-check, if necessary, before committing. People who understood the gravity of the situation, and who didn't phone it in if they weren't feeling it that day. If I demonstrated that I wasn't one of those people, I would fully expect to be fired.