The real-world analogy I use is that of a brick-and-mortar business like Walmart.
Walmart is free to open a business and allow people off the street to shop there, browse their wares, and buy stuff if they want. They cannot force people to buy something for the privilege of visiting, and while they could require a "cover charge" to enter, they're probably going to lose most of their customers if they do this. It's Walmart's job to attract people into the store, and then attempt to convince them to buy stuff when they're in there.
Other shops are the same, and small shops are probably an even better example: while most people will probably buy something in a Walmart if they take the time to go in there, if you check out some weird little specialty shop you happen to walk by in a shopping district, it's quite likely you're not going to buy anything in there.
Someone has to pay for that little shop's rent and utilities and salaries, but that's the concern of the shopkeeper, not the customers. If the shopkeeper wants to stay in business, he'll figure out how to get enough customers to buy stuff to keep the doors open. If he fails, too bad so sad. It's not the customers' fault for not buying enough stuff, it's his fault for not having good enough stuff to buy, having too-high prices, having crappy service, having a poor location or operating hours, having an ugly or dirty shop, having annoying workers, etc.
It's no different with websites. It's their job to figure out how to make money on the site, not yours. If they want to show annoying ads, that's their right. If you want to block those ads (just like refusing to look at aisle displays in Walmart), that's your right. If they don't like that, they can ask you to leave, just like a shopkeeper can ask you to leave. But if your browser makes a request, and their website services that request, then they are not asking you to leave, they're inviting you in. As long as sites don't turn away my browser requests, I consider myself welcome at their sites, even with my ad-blocking. If they (like Forbes) refuse to show me the site/article because of my ad-blocker, then I either leave or disable the blocker, depending on my whim (usually I just leave).