The shocker was when I realized that the emotional tone is almost certainly the result of what a few high-status members of a group prefer or preferred

Yes, if you have gained temporary influence over others one of the ways you can put that to further use is by trading that influence into an environment that accords with your preferences.

but the emotional tone is generally defended as though it's morally superior

Regardless of how it comes to be established as a social norm, it could be that a particular tone is more suited to a particular purpose, for instance truth-seeking or community-building or fund-raising.

(For instance, academics have a strong norm of writing in an impersonal tone, usually relying on the passive voice to achieve that. This could either be the result of contingent pressure exerted by the people who founded the field, or it could be an antidote to inflamed rhetoric which would detract from the arguments of fact and inference.)