At the risk of simplifying some things and complicating others, I'll draw a parallel to television and VCRs. With regard to broadcast channels (or unscrambled satellite channels), the signals are available, with the right equipment, cost-free.

You could say the same about much web content.

In the case of broadcast television, the costs are obviously underwritten by advertising content -- and occasionally you'll see banner ads on some of the more forward-thinking web sites out there.

Some VCRs and VCR-like devices have mechanisms to bypass commercials during playback or in pseudo-real time. Besides that, you can always change the channel, leave the room, check your e-mail, or just press MUTE on the remote control. There is an implicit understanding that a non-zero portion of watchers will not ignore the commercials, but the option is to view them by default.

Compare that to ad viewing on web sites. By default, browsers do not block ads and they do display images. Installing a filtering proxy or disabling image autoloading bypasses this, as does using a non-graphical browser or an automated process through LWP. Again, there's an implicit understanding that, by default, a non-zero portion of browsers will see the ads.

Just as with television, there are no requirements that a human will actively see the advertisement.

Having said all of that, I would suggest that this applies to personal use only. Certainly rebroadcasting a television program with the original commercials stripped out (or worse yet, replaced with your own commercials) would be rather immoral (and even less legal). The same idea seems to apply nicely to web content. Yes, it's easy to grab the latest UF comic and mirror it on my web site (and that's nicer than linking it right off of Yohimbe's server, but still unethical), but as the company derives some income from ad revenues, I would be depriving them of that income for other people -- by default.

If web sites make certain aspects of their content available, say through an RSS file, that would be fine. Anything else? Personal use seems to be okay, but public redistribution is right out.