The progressive idea that you should only cast minority actors to play those minorities has a two-fold motivation. It is partly an employment issue, a push-back against a previous open-to-all tradition that meant there was less work for minority actors. But it is also, and I would say at its core more so, about respect. There is something disrespectful, the reaction against Halle Berry and Scarlett Johansson suggests, about casting a cis actress in a transgender part.
