IMHO this logic is a old wives tale/fools errand unless you live or are parking a car in a place that is so f***ed up that you know with near-certainty that your window will be smashed. And if that is the case, you probably shouldn't be "there".
Additionally I've had friends "there" who did what you did and still got their
window smashed.
Even in most big dump cities the overwhelming majority of car thefts are from unlocked doors. There's an entire class of thieves (probably the majority) that aren't willing to bust windows because of the amount of time, noise, etc it attracts and the cost/benefit payoff. Not to mention on a lot of cars, you work the lock from the inside (via the now broken window) and now in addition to all the commotion you created by smashing the window, now you're setting off the car alarm to open the door. Average hoodrat druggy guy at 3 am in big-dump-city-outskirts doesn't want any of that shit. He'll just walk up and down the street (or wherever) and test door handles till he gets a car that opens and then just quietly take shit.
That having been said, I don't approve of the kopsch f***ing with peoples cars and hanging love notes on them, regardless. I don't understand how that really
is going to help anyone. The people that left their shit unlocked either A. did it on purpose or B. are so absent minded/ADHD/space cadet etc. that they would never remember it in a million years anyways, even if they got robbed like a week before that tag got placed on their door.
-Mike