It is neither arbitrary nor moving. If one person has power over another in a corporate context, the senior person should not date the junior person.

A pretty simple way to think about it: who in your company would you be afraid to be frank with if you knew it would upset them? Those people shouldn't be allowed to date you. That leaves billions of people for each of you to date, so I'm not seeing a big problem.

Are there edge cases and nuanced situations? Sure. Is that true about any important rule? Sure. E.g., look at your company's conflict of interest policy. For the sake of clarity, a bunch of relatively arbitrary lines will be drawn. But that doesn't produce the same hand-wringing and rushes to the fainting couches that a rule like this does. I'll leave the reason why as an exercise for the reader.