It's not just closing the barn door; it's an admission it was open. This wasn't even a "reminder: this is contrary to our CoC". As "Effective immediately, Crew Scheduling is now only able" implies that they were able to do more previously, as if they had a policy of booking DH crew after boarding. It will be interesting to see if the remaining US Carriers issue similar memos.
Ian W can go on about "The Chain of Command", but it's a jet parked at the gate before the captain accepts it.