Was the weather stripping installed by the manufacturer? If so it would have been his responsibility to seal the bottom before the strip was installed. The installer would not have taken the weather stripping off to seal it as most times the weather stripping would be destroyed or damaged depending on the type it is. If the installer did install the weather stripping and paint the door it would be his responsibility to be sure the bottom of the door was sealed. If the installer installed the door and weather stripping but a painter painted the door, then who's fault is it?