It's been explained to me like this, and knowing the Inyo this is one of probably 3 or 4 explanations.
Although you are not required to have a BRC in the Cottonwood drainage you need one on The Main Trail, therefore, it is a requirement when you leave Horseshoe Meadow.
However, you can get a special dispensation from the Lords of the Inyo if you promise not to camp east to Trail Crest and go directly to Whitney Portal.
Bill