Well it would be unethical when it resulted in harm, I guess. That's the usual basis of ethics. And I can certainly see that stripping out the way sites such as El Reg make money but still taking their product harms the site. Yes, it's not enforced by a contract or DRM, but I don't necessarily take the position that anything not actively locked down or forbidden by contract becomes okay to do. On the whole, I am fine with sites having advertising and I respect that I am getting something from the site so I have the courtesy to not block them getting something from me for it. Legality is of little interest to me as in my experience the law often has only a passing acquaintance with ethics (and sadly, the entire history of government in this species will probably back me up on that :/ ).
Where I do feel entitled to block it, is when it starts spying on me. That is an actively aggressive act which I reject. I do not appreciate that to use the Internet I must submit to giant ad corporations knowing everything I do and everywhere I go.
So in short: I'm fine with advertising and wont normally block it because I want the sites I like to be profitable and their employees to be paid. But I despise tracking and will cheerfully do what I can to stop that, even giving up the odd site (such as forbes.com) that make it too difficult to avoid.