Yes, in the U.S. it is required to release open applications for any position like this...
I'm pretty sure that's incorrect. Private companies are not generally required by law to open applications. They may be required if they are federal contractors, or federal money is involved in some way. But more likely it is an internal policy at work.
But an open call for a position that is intended for a particular internal promotion is normal here in the U.S.
Not saying this is the point for WotC - but in general, if you hold an open call, accusations of discriminatory hiring practices are harder to make stick.