bbPress.org

It’s a limitation of WordPress.

bbPress does not use anything not native to WordPress. All the functionality is accomplished with custom post types, taxonomies, post meta, user meta, etc – it works really well.

However there are some features, such as this, that just can’t easily be done within the confines for the WordPress schema. I’ve talked to JJJ before about this, and it’s just not something that can easily be done without performance hit or using custom tables, neither of which are really ideal for bbP core.