The reason this occurs is that PokeBase, Meta and RMT are all separate ‘instances’ of the same platform, which are independent of each other besides using the same database of users. This means that the only thing transferable across sections is data specific to each user page, e.g. your wall posts, profile, etc.
However, since posting on walls is a points-based privilege and not directly linked to your account, it isn’t transferable across sections. Yes, it would be ideal that earning the ability would make in functional in all sections, but due to this quirk in the way the community is set up, it’s not possible without overhauling how the software works.
Technically the site admin Pokemaster can change this, but I can’t imagine it would be a priority for him right now.