Ineffective Security

I bought a new edger at Home Depot. They didn’t demagnetize it correctly, and it went off as I was leaving. This was my only item. The door was right in front of the cashier, so I kept walking. I figured she remembered I had just bought it from her six second ago.

No. She called me back. The manager, who had seen the entire thing, beckoned me over. He took the edger and demagnetized it. Then I left.

How did this help anyone? The point of the security system is to let them know when I’ve taken something without paying. But both of them saw me take it and both of them know I paid for it. How did demagnetizing it help anyone?