Now the thing with websites is, there is no contract. You just ask them for some content and they give you the content + ads. I'm free to ignore the ads. A good parallel would be a local newspaper that gets send for free to everyone, but also includes ads. I'm completely free to ignore the ads in this case. I would also be allowed to build a sorting machine that sorts out the ads before I ever have a chance to look at them. That's obviously no theft, after all they gifted it to me!
And this is exactly what most websites are. They send me free content and include some ads, without any contractual obligation. If they don't want me to just see the content without looking at the ads, they can always set up a contract requiring me to look at the ads. Or only show me the content after I've seen the ads. Or do anything else, that isn't just sending me content and ads without any obligations.
But until they do that, it's definitely morally right and legal to sort out the ads, just like I do with my local newspaper.
And something that I want to touch on seperately because it really annoys me. It's completely ridiculous to speak about "theft". Theft always requires actively taking something. The taking part is really important when speaking about theft. But I'm not taking something from websites, I am asking for it and the website freely gifts it to me. On HN I'd really expect people to know how HTTP requests works. I'm not evading access controls, I'm not forcing them to give me the content, I'm not sneakily taking it away, I'm not hacking into their systems to take it.
I ask via the official way to ask for the content and they decide to send it to me. What I then do with the content on my local system is completely irrelevant and I have to right to modify it however I want, as long as I don't make it public or use it commercially obviously.