This is why my stereo will be locked in a console and speakers/amps will be secured to something with screws. Luckily, I never had to learn this lesson but someone actually cut a hole in my brother's Jeep soft top and ripped his stereo out of the dash and ruined the dash on top of it. We live in a small area so if that will happen around here then I'm going to take every precaution I possibly can.
I am terribly sorry to hear this happened to you, though. It's truly disappointing that people will do things like this.