It makes sense to use Smackdowns time promoting the RAW title match if the wrestlers in the RAW title match are Smackdown superstars. Until one of them actually wins the RAW belt, Charlotte and Becky still on the Smackdown brand and still showing up on Smackdown.
That said, they seem to have impied that the womans tag tiles are float between brands so in theory its possible for Bayley and Sasha to show up on Smackdown too. But their first feud appears to be with RAW superstars.