Phil Again wrote:

Short Answer: Lawyers and (product) Insurance.

Too Much Information (TMI) answer:

NEMA, National Electrical Manufacturing Association sets the standards
for all US Electrical plugs and receptacles. Search for a Wikipedia
write ups on NEMA standards. There are links at the bottom of the
Wikipedia to NEMA configuration charts. Your plugs are NEMA 1-15 type
plugs.

The National Fire Protection Association publishes the National
Electrical Code. The NFPA has adapted the NEMA standard as part of its
new building code for Electrical Safety. Many states adopt the NFPA
electrical code standards in their building code and statutes for
enforcement of such building codes.

Underwriters Laboratories, inc, the testing agency for Insurance
companies that issue policies against product liability, adopts the
National Electrical Code (which includes the NEMA standards) as part of
its product safety testing and check list.

Thus, in order to purchase product liability insurance a maker of
consumer products, like a homeowner's hand drill, needs to submit the
product to UL for safety testing. UL will give its blessing only
provided ...... (yada, yada, yada)

Thus the products you buy will have a narrow (hot or black wire) and a
wide blade (neutral or White wire indicated by the "W" on the NEMA 1-15
standard.) Even if the general public safety intent and need for the
narrow / wide blade makes no difference in a specific manufacturer's
product.

The NEC is written by panels that include many interests -
manufacturers, engineers, contractors, labor, affected industries. UL
and the NEC try to be compatible (but I can think of at least one place
they conflict). As far as I know, the NEC has no standards for
receptacle configurations except RVs and trailer parks.

UL develops standards for products from different panels of
knowledgeable people. It is independent from the NFPA and NEC, but
people involved also want UL standards and the NEC to not conflict. UL
does not adopt the NEC.

UL standards are for product manufacturing.
The NEC is primarily an installation standard.

The NEC has limited product design requirements.
Equipment installed must be "approved". Approval is strictly the
decision of the "authority having jurisdiction" which is typically a
state or municipality. The AHJ usually wants products to be "UL listed"
(or maybe listed /labeled another reputable lab which probably tests to
UL standards).

NEMA is a manufacturers group. It sets a number of standards. The
question is how those standards become requirements for equipment. NEMA
receptacle configurations may be included in some UL standards, I
don't know. UL could also just have performance and design standards,
for example plugs must be polarized, with specific configuration
selected by manufacturers.
(NEMA categories for enclosures have been adopted by the NEC.)

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