Actually, the wording on the RCI Exchange Plus form on the subject is pretty specific that "Weeks associated with the RCI Points Program are not eligible for exchange through the Exchange Plus Program" rather than "Weeks at a resort that is associated with the RCI Points Program...." It's more similar to the limitation on Wyndham PIC-eligible weeks, which is open to weeks at a resort that's associated with the RCI Points Program, but not enrolled in it.
The other thing to consider is that RCI, Club Wyndham, and WorldMark are all Wyndham Destinations products, so allowing weeks that could be in the RCI Points program to be used in Exchange Plus for WorldMark or the Wyndham PIC Program isn't really taking business away from Wyndham Destinations, but instead a different pair of methods for Wyndham Destinations to obtain the inventory for rental (for a fee) from owners that don't want to be in a continuous program like RCI Points. Not really sure about what the restriction is for HICV and weeks in that sort of program is, but it appears to be more of a contractual relationship between RCI and HICV rather than something RCI can do unilaterally or to sister-components of the same corporation.