If you post it on a public network, it is defacto, public. If you don't want it scraped, take it down, or put it behind a login. If the user provides the login to a scraper, then the scraper has permission.
At the risk of bringing out the "you wouldn't download a handbag" brigade - you could make the argument that just because I leave my bike unlocked on the street doesn't give you a right to ride it off. Even if you go for "forgiveness rather than permission", if you ride it a second time after I've told you I don't want you riding it, then you're in the wrong. Obviously, there are philosophical arguments to be made ab…
If you have an issue with that, you should be moving your bike somewhere people can't take a photo from the public street. Not have someone creatively interpret a law that says where I am is suddenly not public property, because you asked me to stop using my camera.