There are no laws that govern the sale of electrical materials so the big box stores do just what they need to do to stay legal. They advertise expert electrical ( ambiguous ) help and not licensed electrical help from their sales people.
There are laws that govern the installation of electrical systems and this is where the homeowner takes full responsibility. Most of the week-end jobs would actually require a permit and an inspections so it's the homeowner that chooses to use the materials in an incorrect manner.
Think of it, you can buy yourself an assault rifle without one minute of training and what you do with such a weapon is up to you ( all you need is to meet legal requirements, normally breathing)
When a person buys electrical materials they have made the decision that they will be responsible for the use of said materials just the same as if the were buying a weapon.