Those scales in the picture just scream “prove you’re not a criminal, scumbag”.

So what? This seems like such a weird thing to be concerned about. Do you get upset when you are asked for ID when you buy alcohol? Or when a police officer asks to see your driving licence?

why would anyone opt into this if you can go to a different store instead?

Because weighing your trolley only takes a second and still saves time over scanning things manually?