I think people are intentionally missing the point.
Someone makes a website.
The time it took to make the site costs them money either directly or indirectly (they made it, or they paid someone to make it).
Someone is paying to keep the web server online in bandwidth, hardware, content upkeep or software costs.
The only way most people can make money from a website is to show ads. Ad companies can tell if their ads are displaying and pay less if their ads are blocked. The only way for people using this model to pay for content managers, bandwidth costs, faster servers, etc is through ad revenue. If site owners don't get paid, they can't pay for these things, so one or many of the things running the site don't work as well.
If you can't afford reliable content managers (or you yourself have to work a real job because you don't get paid enough), the content suffers. If you can't afford a lot of bandwidth, the site gets slow from throttling it. If you can't afford up to date anti-virus (or a good ops guy to manage your firewall), your site is easier to hack and take down. If you can't afford a new nic card (or F5 for large sites with server farms), your site goes offline with hardware issues.
If a large business owns a site and it doesn't make money, it simply takes it offline or invests less in the above mentioned maintenance costs until the value of the site is diminished to the point that it's better to read another site - or a magazine for that matter.
The thing the guy is trying to say is that if you like the current state of the site, it takes money to maintain. If it doesn't make enough money, he doesn't have to work for free. If you don't care if the site goes down or degrades in some way, go ahead and block the ads. If you take a "I wasn't going to pay for it anyway, but will if it's free and those ads are like a tax on my sanity so I block them" stance, what he's saying is that you're reducing his ability to make money from his site and by extension, lowering the overall experience for everyone.
I worked for a news site that made money with a per-view ad model and can tell you that it takes several million dollars a month to maintain a world class news site. The AP must be paid for content. Editors to moderate the AP must be paid. Production Operations guys, Test Operations guys, Developers, Release Engineers, Project Managers, Ad Operations, Managers for PM/Dev/Editorial/Test, Marketing, Sales...all have to be paid.
It's always a delicate balancing act with your corporate overlords who want to make a lot of money (to pay the bills, and appease THEIR corporate overlords) - while trying not to alienate your user. Big invasive ads make more money per impression than little ones that few people see. That you don't see giant ads on a given site all the time is a testament to their restraint or ability to ward off the bottom-lining execs.
I love free sites like Slashdot, but they're probably has high quality as they are because the majority of people let ads display. Sure, Slashdot would probably still be on the web if nobody viewed the ads, but it's unlikely to have a lot of the features that ad revenue paid to have developed.
I'd be interested to hear what Slashdot would be like if they made no ad revenue from CmdrTaco. Would they have been bought up by their corporate overlords? What would that have meant if they hadn't?