This quote sums up nicely why Diebold should not be trusted to secure election machines:
David Bear, a spokesman for Diebold Election Systems, said the potential risk existed because the company's technicians had intentionally built the machines in such a way that election officials would be able to update their systems in years ahead.
“For there to be a problem here, you're basically assuming a premise where you have some evil and nefarious election officials who would sneak in and introduce a piece of software,” he said. “I don't believe these evil elections people exist.”
If you can't get the threat model right, you can't hope to secure the system. [Schneier on Security]