1. There are currently several kinds of wall power voltages and plugs/receptacles commonly in use the US. Notable examples are the various 220-240V NEMA receptacles and the various 110-120V receptacles such as polarized 2-prong, unpolarized 2-prong, and 3-prong/grounded.
2. Although there are code requirements (e.g requiring that new outlets be grounded and polarized, or requiring ground fault protection in bathrooms) it doesn't appear that they mandate a particular plug or receptacle design (?) Though the NEMA (trade association) developed plugs can and do conform to the code requirements.
3. Power cords and on-device sockets aren't entirely standardized, but it's not nearly as big a deal as if there were dozens of 110-120V wall socket types rather than the three in common use.
And the intent of the law is to bring the uniformity that exists in wall sockets to the on-device sockets.