It used to be that you needed a key to lock or unlock a car door. Those days are long gone, and rare is the car that does not have a remote control to lock and unlock the door. This is not a problem. I am the first to admit that I like my little electronic valet to have my door unlocked for me when I arrive at my car.
The problem is that in the interest of saving money, car manufacturers have decided that it is fine to use the horn instead of the little friendly chirp to let you know you have locked/unlocked your car. Horns are meant to serve a purpose and that purpose is to quickly and clearly get someone's attention, to alert them to an imminent problem, or more likely to get their attention in order to avoid a problem. Often it is to avoid a possible accident. In order to accomplish these goals a horn needs to be loud and attention getting: "Pay attention NOW!"
Confirming to you that your doors are locked and your car is secure does not require a loud or attention getting signal. Car manufacturers know this, as originally this message was delivered by a friendly chirp that said, "Everything is fine, your doors are locked and your car is secure." Sometime in the past, a conscientious automotive project manager had the ideas to save a few dollars by using the car horn to confirm that the doors are locked.
The problem is that cyclists are riding close to parked cars and do not have their "windows rolled up." When (an appropriately) loud horn goes off as you are cycling by, it is at best upsetting, and at worst dangerous. Until you have experienced it, you can not imagine the fear of bicycling along in traffic and hearing a horn honk right next to you. You don't know if you are in danger. Is something coming towards you? Are two cars near you about to crash? Is someone warning you about a danger? No, it was just somebody locking their car. How infuriating! I could have swerved into traffic and been killed. My heart is racing. What was wrong with the friendly little chirp?
By the way, bicyclists aren't the only ones in danger. Motorists can also be confused / startled. Pedestrians can as well. It also has the negative effect of "The Boy Who Cried Wolf." Eventually people will start to ignore car horns, not good when there is a real emergency. Do you really want to have someone ignore your horn as they start crossing a street in front of you?
I know most people who read this will not be regulators or automotive product managers, in a position to change this or regulate it. But if you ever buy a car, you can at least help to make the world aware of the issue. Ask the person that is selling the car (dealer or individual) whether the remote has a chirp or a horn. Let them know that you prefer buying a car with a chirp. People can't fix a problem that they are not aware of. Let the world know this is a safety issue.