Corporate policy that implements regulations would be my guess. Or, in this case, does not.
Other countries are far more into preemptive enforcement of regulations than the USA, but even there things tends to only get better before they get worse. Tesla Grünheide has already racked up at least one amputation, and that's a factory, not a place where they invent new kinds of rockets.
But regulations don't say "fire this person". They're normally far more high level than that, like, "when something bad happens, trigger a corrective and preventative actions process and write down what you found and what you're going to change, if anything".
Effective regulations very much do put the employer on the hook, no way to accept something like "voluntary employee risk-taking" outside of in some cases not looking for it as hard as they could. Employers are at risk of getting fined for tolerating (or worse) employee risk-taking and if those fines are sufficiently high they will certainly tell the employee "stop doing that that or else". And the last stage of "or else" certainly is getting fired, perfectly in line with other worker protection laws that might be in place that would otherwise prevent that getting fired.
More like:
'we have a policy to follow regulations, you have blatantly broken regulations that has now put us at risk, thus you are non-compliant with our policy to follow regulations, so good-bye'.