There is such a fine line between ethics and rules or laws, and expectations for ethics in the workplace often blur that line.
Essentially, ethics are based on personal belief and morals. What one person considers perfectly acceptable behavior may be completely off limits to another.
When a company spells out what is and isn't acceptable, it could never take every potential issue into account and it is really starting to tell the employee how to feel and think.
At this point, ethics become rules, and, even so, enforcing those rules may be next to impossible.