Amazon Go

Maybe they also weight you when enter and leave the store. They can then cross check your weight with the weight of the items you have bought.

You just had a strike of genius

I can't tell if your comment is tongue in cheek or not, but I highly doubt they will make people weigh themselves when they walk in. If they aren't confident in their image recognition technology they would improve it rather than throw in an additional method of tracking.

They could weight the contents of the shelves. Based on that you may know what you took out. I actually looked into that for a retailer.

Many systems already do this for controlled access. Would be a useful bit of data to have if easily implemented. Problems are when access is not 1 by 1. You could get useful, but not consistent suggestive data.

and if you use the restroom?

You get a receipt. (hat tip Douglas Adams)

I'm pretty sure you can weight persons when they pass the entrance and exit automatically, by adding sensors to the floor.