Voting machines that use proprietary software should be illegal.
Well, there is an argument to made that open source software would let all the hackers easily determine where the vulnerabilities in the software are. However, the flip side of that is that there would be 10 times as many people who are not hackers looking for those same vulnerabilities in order to secure them.
But either way, whether the code itself is open is less important than how the code works. There are standards for accounting software that have been in place for decades which make it nearly impossible to hide fraud without leaving evidence behind for the auditors to find. And counting votes is basically a very simple accounting functionality. So why are not we not just using the template of an accounting system which has a verifiable audit trail for every transaction to tabulate votes? The only reason I can think of is that nobody wants there to be an audit trail.