Ads are your payment to the site, if you are blocking ads your not giving your fair share of payment.
Think of it as a restaurant offering a loss leader (like an entree sold at below cost) to attract customers to come in and order the main course (which is where they earn). The loss leader is the free content on the website which entices people to come in and visit the site, while the main course is the advertising which brings in the bulk of the revenue.
However, unless the entree is specifically bundled with the main course as part of a set meal, there is nothing stopping a customer from ordering just the entree, and leaving without eating the main course. Sure, the restaurant makes a loss, but that's just all part of business. You can't accuse the customer of cheating the restaurant this way. They set the stage to entice me to act a certain way, but by no means am I obligated to behave in the manner that the restaurant wants me to.
Same here. The site owner hopes that by visiting their website and viewing their content, I also view their advertisements and earn them some revenue. And some websites do in fact go to the extreme of forcing me to disable my ad-blocker before allowing me to view their content (in which case I just leave and never come back).
I don't know if all this talk of ad-blocking as raised my awareness of not, but I find ads on my phone more and more irritating. Scrolling through articles on Reeder on my iPhone, it disgusts me how much screen estate is taken up by advertisements. Scrolling lags. I want to click on a link, an ad suddenly loads and I click on it, taking me out of the page and forcing me to reload it.
iOS 9 couldn't come fast enough. I can't wait for native Safari view in 3rd party apps finally offering me all the browser goodies like reader mode (which strips out all the non-content distractors), icloud keychain support and (yes!) ad-block for everything else that gets through.