I have read a post on Hikashop forum stating that a registration via their checkout would also take care of a registration to CB
Are you aware of this ?
It's possible they've integrated their registration with CB. We've no specific integration with Hikashop though.
Anyway, even if it's true that would mean to turn joomla user registration on. And you think that's not a good idea ?
As long as CBs system plugin is enabled you'll be fine as it'll redirect away from core Joomla user views to prevent registrations directly through Joomla.