But a BB gun specifically comes with a warning not to shoot people because it might cause harm, not to mention that regulations around fake and/or air guns are pretty hardcore already.
To maybe give another example - as a consumer I shouldn't care if a phone I'm buying was made using child labour or not - I expect the authorities in every country involved in its production to enforce laws against child labour.
I eat canned tuna also, but that logic doesn’t work generally.
By the same logic you could say that if you could buy a BB gun, it’s ok to shoot people with it, because it must not be harmful.
Come to think of it, that’s what kids did to each other when I was growing up.