Connecting to the Public Internet, serving content to any and every connection, without login, means you are effectively broadcasting your content and have no legal basis to impose terms. This is why scraping is legal; the only defense against scraping is to put up a login wall - Gate Down. If the gate is down, you can impose your own conditions and agreements with the binding legal force of a contract.
"By continuing to use this site you're agreeing to blah blah blah" is cute fiction and has no legal bearing in the US. The internet works the way it does because it's empowered and protected by law and convention.
I have the right to manage traffic to and from a device I own. If your server sends me traffic, it's communicating something. If it sends me content on request without my having entered into a contract, then the default case is that I'm entitled to that content, full stop. It's up to you, the provider, to decide and manage how your content gets served, and whether you want to participate in the default mode of public access. By not incorporating a login on a server you connect to the internet, you are choosing, defacto, to participate in the public access mode of content serving, and are responsible for doing so. You are expressly granting anyone connected to the internet permission to download that content, making them entitled to it by the mere fact of connecting to the world wide web, in the same way that projecting a photograph on a public wall is expressly granting the public permission to view said projection, or by speaking in public, thereby granting the public the right to hear what you say, and so on, and so forth.
I'd hazard that a vast majority of users of the internet are completely ignorant about what servers and clients are doing, and the laws and systems which govern them. I can choose to block signals from ad servers, and i can choose to access content from a server over protocols which it's prepared to serve. Youtube does this; it's too valuable for them to remain gate up, thereby leaving third party clients and other methods of access on the table. It's perfectly legal, respectable, ethical, and moral to skip ads. I'd even go so far as to say it's immoral to participate in adtech, so by avoiding it, you're actively making the world a better place, even if it's only yourself.