It's a stretch to assume the employee "went rogue" in the sense of crossing a well-defined boundary. I'd hazard a guess that the no-poach rule was phrased (or at least understood) more as a suggestion, since explaining the reasoning behind the policy would hand HR evidence of the illegal cartel. Perhaps they even gave the HR people a BS reason, like "poaching leads to salary asymmetries and conflict in teams," which the unknowing HR employee proved wrong under the assumption that she could successfully work around the stated issue. When the no-poach suggestion didn't work, they needed to resort to making an example of someone to get the subtext across using the time-honored strategy of creating a "obey me without question" atmosphere to avoid uncomfortable discussions.