Youtube is not free - ads and you're giving your data away. C'mon mate, you have to willfully be ignoring the fact that ripping the songs off of Youtube as opposed to listening to them online cuts the revenue stream.
Many sites do refuse to serve content on detecting Ad blocking but because you're not making a "copy" and storing it for later use, the websites can't sue you - only refuse to serve. If you "hack" the refusal to serve, they could potentially sue but generally won't bother given the economics.
In this case, downloading the offline copy gives them a leg to sue - and even then it is more economical to go after the enabling website.
What if I mute the ad? What if I minimize the browser window while the ad is playing? What if I go to the bathroom while the ad is playing?
Are any of those also conceptually the same as theft? If so, where do you draw the line? Is it wrong to mute commercials on OTA TV broadcasts? Is it wrong to change the channel when a commercial comes on? Is it only ethical to change channels between shows, when the ads are not based on the current show? Is it wrong to tear ads out of a magazine, since we all know the cost of the copy doesn't cover the cost of producing it?
Until you can answer these questions, you can't claim ad blocking is conceptually the same as theft or in any way illegal.
The bottom line is that, every time an advertiser purchases an ad, they take a gamble. It might be that no one sees it, or that it has no impact. Content freely offered obligates no one to watch any part of it, including any embedded advertisements. If they want to enforce restrictions, they must restrict and sell access to it.
