Its considered a bigger risk to the company, to disrupt the executive suite than the rank-and-file.
That was an example process. Come up with one! None are perfect. But I know certainly, that a company with a pragmatic process will be in business longer than an ivory-tower non-process (Ban work relationships! Punish employees routinely!)
This isn't just about punishing people, though. In fact, to some degree it's not about that at all. It's ultimately about the company, not the individuals involved. It's about preserving stability and minimizing risk. Moving a manager could be much riskier for the company than someone lower down in the org chart.
And part of the justifications for these rules is to prevent Involuntary relationships. "Fuck me if you want a raise next year."