Honestly what went on there? Nearly a year has passed since A Thousand Sons came out and I still don't understand what happened.
Was it purely a case of the author taking liberties with the fluff and ending up with something that didn't really make sense or was it something more deep and meaningful?
Lets look at what happened
The Emperor ordered the legions together to discuss the psyker problem. Now in the original fluff I read this was purely a ban on the sorcery performed by the Thousand Sons legion. Now its apparent that its a ban on all psykers. We see this in the first of the Garro audio books, an Ultramarine librarian is shunned by his brothers for using the powers of the warp. So why so Space Wolf Rune Priests still funnel their psyker powers at the end of A Thousand Sons?
Its a sticky part of the fluff that I hope is eventually cleared up by the writers.