I assume you are referring to web crawling? When you put up a web page, your intention to make it public is pretty clear, barring a mistake. Your intention to

I assume you are referring to web crawling? When you put up a web page, your intention to make it public is pretty clear, barring a mistake. Your intention to have it crawled and indexed is a lot less clear, but you usually don't get a lot of say in how people consume things you make public. So I think opt-out makes sense for that. When you put up a wireless network in your apartment, your intention to make it public is a lot less clear. You are putting it in public, so the argument for opt-out is at least coherent. But I don't buy it because it's clearly not most peoples' intention.