Video Doorbell Battery Low at Install

History: I recently bought a Video Doorbell ($99) and went through the installation process. The doorbell chimes my doorbell but gave me anotification the battery is very low. Right out of the box. Question: Why do I need to plug the Video Doorbell into the wall outlet to charge (fully) if it should be getting it's power from exisitng doorbell hardwire? That is what customer support told me I need to do before I go any further. In the future, it should get it's power from existing hardwire, correct ?? Any help would be greatly appreciated.