@AC (What ads?)
You didn't pay to access the WWW, you paid your ISP to route TCP/IP traffic. The content on the other end may be "free" (access without monetary donation) or not, but you don't automatically get the right to access it.
Consider this; The revenue from advertising is what keeps the site free. Hosting, bandwidth, and maintenance of content are not cheap. It takes time, effort, and sometimes considerable personal investment of funds. By disabling advertisements, you take a little more of their income away from them, no matter how small.
Next time you go to your petrol station, tell them that you've already paid for your car, so you don't want to pay for fuel to run it, or that the fact that the majority of the energy released in combustion is heat means that you are only paying for the percentage which is used in motion. See how far your "I've paid once, I'm not paying again" mentality gets you.