When you say "That is not how HTTP works" it suggests that your claim is that anything that HTTP allows is ethically OK to do. However that is clearly a ridiculous stance, since a DDoS attack is a stream of valid HTTP requests and that's clearly not OK.
So I'm left wondering what your argument actually is for why unwelcome scraping is ethically OK.
I find this an interesting question, because while I would love for protcols to also define ethics, I feel that would be scope creep for the poor protocol designers. There's a wide variety of conduct and ethics questions that a protocol cannot address.
Where I myself draw the line is at protocol behavior intentionally designed to obscure my intentions. For example, sending my requests from a wide variety of IP addresses is behavior that is specifically designed to obscure where I'm coming from; my only intent in doing so would be to circumvent the intent of the serving machine from providing lots of content to a single requestor. At that point I'm engaging in deceptive behavior; I've crossed an ethical line.