Van Buren violated policy. He should have been terminated. Van Buren was prosecuted for a criminal act not narrowly reflected by the law he was prosecuted against. The SC majority opinion mentioned exactly this.

> Am I missing something? To me, this ruling means that if a person is granted technical access to a computer system, then that person cannot be held criminally liable for anything they do with access to that system, even if the owner explicitly prohibits it.

Yes, you are confusing a policy violation for something more grand. I suspect most people are confused on this matter due to a selection bias favoring criminality for a violation of public trust.