Some people would like us to think that it is. Their argument is "You agreed to look at ads, in exchange for seeing the content on our site, and now you're reneging on the deal. So it's theft."
First of all, I did not agree to look at the ads on your site. I didn't even agree to let my browser display the ads on your site. I'd say it's the other way round - you're shoving unwanted ads at my computer, which costs me bandwidth. So you're stealing, or at least attempting to steal, from me.
I do understand that you've decided to use a business model whereby you think that your content isn't good enough for you to be able to charge for it, so you'll get money from adverts. My suggestion is that you should improve your content sufficiently that I'm willing for fork out a few dollars.
And I have a very real problem with adverts. The one and only time that my computer has been infested with a trojan, was when I was accessing "The Register", an excellent UK-based tech news site. But they outsource their advertising, and my computer (running Windows 98) was infected by something it got via an advert.
After half an hour trying to get rid of it, I decided to just reformat the drive and reinstall. And then I thought, while I'm doing this, I won't reinstall Windows, I'll put Linux on.
And they aren't the only ones.
It's called "malvertising". In 2012, it was estimated nearly 10 billion ad impressions were compromised by malvertising.
So why would I open up my computer to this risk? I mostly tend to visit well-known web sites, which I would tend to trust. But the attack vector isn't the web site I'm intending to visit (such as The Register), it's the adverts.
And that's the first reason why I block adverts (I use Adblock plus). And I block flash and java, also to improve security. And I also have a long list of sites that are redirected to 127.0.0.1 via my hosts file.
The second reason, is bandwidth. Modern web pages include a lot of stuff, because they're trying to make them look "designered". They try to make them look like magazine pages (and sometimes in doing that, they designer them so much that my browser can't properly render the page, and in extreme cases it crashes). If you add the often large adverts to this, then you get web pages that take a lot longer to load than they need to.
And the third reason is that the ads won't do anything even if they display on my browser, because I'm not going to buy anything from them. If I want something, I'll usually go on Ebay (or a few other useful sites) and find it there.
I've heard people complaining about Facebook ads too. Again - I just don't see them. This might be because Facebook has analysed my thoughts, and has realised that it would be a waste of their time to push ads at me. But it's more likely that one of the things I have running, is blocking their ad.
And it isn't theft. I know you want me to buy your products, but my lack of desire for the products that you want to sell me, is not theft.