Right but it seems some machines are connected to the internet (seriously).

And it's probably easy in the firmware to check if you're being tested (ask Volkswagen). And even if it's not, you could write a firmware you could trigger with a specific ballot.

The bottom line is firmware for these things should and must be open so they can be audited by experts. And hackers must be given access to them so they can try to do their worst.

As long as you treat it as a black box, I'm afraid to say you aren't trusting the device, you are just having faith in it.