It is intentional, and there are plans to change this behavior. The forums were initially designed so that they are treated separately from each other. That is, each forum is standalone. Some things are shared, such as accounts and categories, but for the most part it is like they don't know anything about each other.
That was a decision made by our previous forums developer. I personally prefer to have everything connected. At some point in the future I will be doing a large update to the forums that will resolve all of the disconnects between the various Stardock forums. It should make the sites far more consistent.