What you say is not quite as true as you think it is. I've seen AP guys get pretty aggressive with thieves. Cashiers are trained to look in boxes and buckets to make sure nothing is hidden in them.
Maybe it's regional, but my store didn't put up with that crap.
A few years ago I was in my local HD here in NH. They had one of the Porter Cable pancake air compressor/nail gun combo boxes on an end cap marked down to $150. The employee said it was returned and there was nothing wrong with it. I offered the manager $100 and he sold it to me including a full warranty.
I brought it home and pulled it in. The air immediately started leaking out of the bottom of the tank where the purge valve was busted off. I boxed it up and returned it the next day.
When I brought it in to the service desk they checked the SKU # to the one on the out side of the box. They were different. The one on the compressor was from two years previous. The box was from two months ago. So, somebody had bought a new one, switched it for the old one and returned it.
I said to the clerk, wow what a scum bag move. She said that was nothing. Stuff like that happened all the time. The worst one was when somebody tried to return an air conditioner at the end of the summer. The box looked new. All taped up. Customer did not have a receipt. When she opened the AC box there were two cinder blocks inside. The guy returning it said: “how did those get in there?” and walked out the door.
Yes it is regional. There are stores with no Loss Prevention staff. The thieves know which ones those are.