There are enough noise codes and ordinances throughout North America that define non-emergency horn honking as a violation - further legislation is not required to facilitate the phasing out of horn honking to indicate vehicle security status and other situations (car started remotely, car lost in parking lot). Federal funding to address noise in the US was terminated in 1982, and responsibility for enforcing noise regulation was left to cities and states. However, local law enforcement bodies have the ability to enforce noise regulations.
Automakers can proactively eliminate horn sounds as audible security status indication because non-emergency horn honking violates safety laws and local noise ordinances, and because non-emergency horn honking is inconsistent with the industry's positions on the environment, sustainability, and social responsibility.
Many noise ordinances prohibit non-emergency horn honking, but allow vehicle status indication under exceptions. But most such laws were passed before horn honking came into popular use as a status indicator. In many instances, the exceptions refer to car alarm noise that occurs when security is breached, not when the alarm is being activated. Most noise ordinances do not specifically address "lock alert" horn honking (with or without a car alarm involved). Interpretation is left up to local legislators.
Some people who have questioned local lawmakers about this exception have learned that horn-honking security status indicators are in fact not excluded, and violate the law. When in doubt, ask. It is doubtful that legislation would allow horn-honking security status indication without mentioning it as an exception in the horn section of the ordinance. Here is a sampling of local laws in North America.