What makes you believe it wasn't obvious to them, or that the purpose of turning someone to the Shadow is to infiltrate him or her back among the Light, and not more simply to make them serve the Shadow and deprive the Light of them?


All we know is that it's possible to do this, that Semirhage find out about it through her experiments with Myrddraal, and that it has been used. We also know Semirhage preferred to break people to her will herself rather than use this method to turn them to the Shadow.


And now we can be reasonably certain Taim has long had, or recently gained, thirteen female darkfriend channelers, most likely BA, to serve him...

And we can theorize the Shadow has decided the time had finally come to turn all the channelers who aren't DF at the BT to the Shadow. , one by one. I don't think the idea is to send them back to the Light so much as increasing the ranks of Taim's faction and clearing the BT of all non-DF channelers in a less messy way than by fighting them and risking one escapes and raise the alarm.