My expectation was that this was a policy written based on experience.
It may be due to some analysis as well. Having done a few failure modes and effects analysis in my time, its common for engineers to identify such modes, perhaps do a bit of testing and revise a design or establish operational guidelines to prevent an actual situation from arising. The ability of an HF transmission to cause a spark in a nearby structure may have been something they observed in a lab.
Sometimes we don't wait for things to blow up before we fix them.
