Actually it may be, if their terms of service say you aren't allowed to use devices such as that. For example, lynx, or some programming language user agent.
That said, I wasn't really specifically addressing legality. There's a legal argument, and a ethical argument. The legal argument is quite clear IMO. The ethical argument is what's interesting in my view. People have justified actions that are clearly against the expressed interest of the content providers and producers in some cases, and I find it interesting to explore the reasoning. I myself do this, as I use an ad blocker, but I've drawn my own line and reasons for doing so, and have my own complicated feelings about it.
Having has this conversation multiple times here, invariable security comes up as a big reason for some people why they feel justified, when they do feel conflicted in other areas. Having a content provider serve their own ads weakens the position of security argument quite a bit, so I'm interested in seeing how people react in that case.
Edit: s/moral/ethical/
If I rip out all ads in a print magazine, or throw away the flyers in a magazine - which I do first after buying a mag - then I shouldn't buy the magazine? (magazines are paid for by ads not by me buying them).
Ethically? Maybe not? I don't know, I'm just exploring the problem space. I understand nobody likes advertisements, but while whether I like something may affect whether I allow it personally, I try not to let it affect whether I think it's an ethical thing to ignore.
What this means in practice is that I use an ad-blocker, mostly for security reasons, but understand that it hurts the bottom line of every source it's effective for. In cases where a site puts up a small notice at the top asking me to not block the ads, I generally do so or choose not to consume the content. This wasn't always the case, but it's where I arrived at.
The whole system is broken, I understand that, but I don't necessarily think increased ad-blocker usage will lead us to the solution we eventually need. That will probably require some sort of legislation, but I'm honestly not sure which interest will get served first, and thus more favorably. I'm slightly afraid it will be the commercial interest, barring some really nasty massive breach of aggregated data from advertising companies that's used to very bad effect, and in that case we lose quite a bit as well.
Well, you've given them permission to try. I doubt they are giving you any such permission, so you're actually presuming quite a bit when you say you're welcome to try as well. This actually matters, because as far as the law is concerned, you don't have any right to to the content beyond what they allow you (excluding laws specific to your submitted content, depending on locality).
I understand ads and ad-blocking stir visceral reactions in people, but really, if you don't like what they are doing, you shouldn't use their service. Anything else is just enabling the behavior you dislike, as even if you are not being marketed to, you are providing a reason for other people to want to use the site that are marketed to.