[blacksmith_tb]

It's partly a bad UX choice by the automakers - as a user, the driver wants to lock their car when they get out. But the car will only let them do that if they arm the alarm with the same action, on 90% of all cars made in the last decade.

[mikeash]

I see no problem with that. There's no scenario I can think of where I want the car locked but don't want the alarm engaged.

Part of the problem is just plain oversensitivity (alarms being set off by passing trucks and such), and part of it is bad design. For example, my alarm will go off if you close the trunk. That's completely nonsensical: when the car is secure, the truck will already be closed, and opening it would be the appropriate trigger. But it does mean that I occasionally set it off in my garage because I left the trunk open, locked the car, then came back and closed it.

[alblue]

FWIW there are times when you want to secure the car but have a mobile participant inside the car (and the car has interior sensors that detect movement). Pets and small children come to mind. There are cases (such as filling up at a petrol/gas station) where you want to secure them temporarily in the vehicle while paying but don't want movement setting it off.