Also, with the possible exception of membership-based stores like Costco, the "guards" at the front of American stores like Best Buy don't actually have any authority to prevent you from leaving without checking your stuff. Of course, some of them are not correctly instructed on this fact and will break the law and illegally try to obstruct your exit from the store/parking lot anyway.

I don't know the absolute stats but in 12 years of adulthood I've never had to show a receipt to anyone in the UK and in about 12 weeks of vacations in the US, I've done it several times. So yeah, it might not be a standard thing there, although it does seem to be more of a "thing" people accept.

The only store of any kind I'm aware of around here that checks receipts every time is Sam's. A lot of places will have "less than friendly" staff by the exit, but explicit security is fairly rare in my experience.