Don’t worry they are not charging you for using WhatsApp they will be charging business first. The way they will be doing this is by charging Business per messages when contacting a customer. Just to be clear whatsapp business cannot contact you unless you have contact them before so there is no need to worry about a “sales chats” bugging you in your whatsapp.
The interesting part is that they will not charge companies if they reply to messages inside a 24hr window only if they delay in replying. This way they are motivating companies to reply fast to customer requests so that customers gain confidence that they will get a quick response when going through whatsapp for customer support.
You can read more in this tech crunch post