Thank you for the suggestion, N_.
The bbPress forum doesn’t use BBCode by default, though there are plugins available for it. Implementing it would require a policy decision and we’d have to consider the potential gains and risks carefully.
Different users employ the <i> (italics) and <em> (emphasis) tags in the comments section, to the same effect, but the forum is a bit more sniffy. You’re right that there are inconsistencies, not least with the way tweets and embedded links are rendered.
Moderators only have access to the bbp style pack options and can’t change the quicktags markup. I’ll have a hunt around for anything that could influence this and pass on your suggestion to the site admin.