Recent work by Kuang et al. has identified a robust long-timescale correlation between atmospheric oxygen levels and geomagnetic field strength over the past 540 million years, together with a plausible physical mechanism involving magnetically modulated ion escape. While that work establishes the existence and persistence of the correlation, it leaves open a deeper explanatory question: why boundary strength should appear as a stable, system-level correlate at all, rather than washing out or leading to instability. This paper addresses that question at a structural rather than quantitative level. It does not propose new physical mechanisms, modify established geochemical budgets, or claim that magnetically regulated escape dominates oxygen mass balance. Instead, it asks a different “why” question: under what conditions can a macroscopic atmospheric oxygen reservoir remain stable over geological timescales in the presence of continuous, irreversible loss at the planetary boundary? The central claim is that long-term persistence under irreversible escape is not generic. In regulated dissipative systems, stability requires continuous boundary mediation that renders loss compatible with persistence. Within this framework, the geomagnetic field is interpreted as a boundary-regulating structure whose varying strength produces bounded modulation of oxygen levels rather than catastrophic depletion. Periods of weaker magnetic field correspond to partial relaxation of boundary constraints, increasing loss rates while remaining within a stable regime. Under this interpretation, the correlation identified by Kuang et al. is understood not as evidence of instability or coincidence, but as the observable signature of regulation in operation. Runaway atmospheric loss is not expected unless boundary regulation is removed entirely for extended intervals, a regime Earth has not occupied during its geological history. The contribution of this paper is therefore explanatory rather than competitive. It supplies a law-level interpretation that clarifies why geomagnetic coupling is structurally relevant to atmospheric oxygen persistence, without attempting to replace empirical models or quantitative geochemical analyses. The result is a complementary perspective that helps situate recent empirical findings within a general framework of stability under dissipation.