I can tell you from experience that delivery drivers do not heed beware of dog signs even when clearly posted. They have some idea they are exempt. There may be more to this story.
The problem is that sometimes delivery drivers and mail carriers are required to deliver something directly to someone’s door. They know those “beware of dog” signs protect the owners from lawsuits, but so many houses have those signs, and the delivery workers and carriers have a job to do.