It's not just a lack of prevention. It's a purposeful, deliberate choice to send this content even if you're not paying or loading the ads.
Closer. Its more like if your friend says you can use his car, but you need to pay him $x per mile. So you use an odometer changing device to make it look like you didnt actually use it.
Maybe a better analogy would be to walk through his lawn if he didn't put a fence up. This probably does about as much damage to his lawn as bandwidth would be wasted on you as an ad blocker.
And wasted bandwidth isn't the real issue, unless the business in question's only expense is bandwidth.
Well then what's the issue? Bandwidth is the only thing I'm costing them really, the only difference between me going there and me not going there is bandwidth. If I legitimately couldn't use the site due to a pay-wall I wouldn't go there. Unless there's some secret way to make me pay for content that I'd only consume if it's free?