Well, you usally have to do a combination of things - like press the button and start the car at the same time. Something like that. You can't just get in, tap the switch and kill the alarm, becasue, as you pointed out, that would be really easy to defeat. Sometimes this switch is hidden and where it's hidden usually depends on the type of car and the shop that did the installation. Call them and ask where they typically hide the switch if its not in plain sight.

I can start my car with the alarm going off - to silence it, I need to start the car with the button depressed.