Am I missing something? If Der Spiegal doesn't want people to view their content while using an ad-blocker can't they detect and block those users (assuming those users also allow JS)?
In my experience as an user, no website has ever beaten an ad blocker. It's like trying to secure a computer against an adversary with full physical access. The content has left the server and is on the user's machine. Nothing can stop a sufficiently determined user from consuming it, with or without ads.
I answered a question on User Experience.SE about this:
http://ux.stackexchange.com/a/96907/3437
I tried to express just how big the power disparity between the developer and the user is. Looking back now, I think I came off as too extreme.
I think the other answers are excellent; they address the UX aspects in a way I wasn't able to. Definitely learned from them.
For example, if you were watching a TV show, it would give you a playlist of video segments and ads, all having random urls (hulu.com/<uuid>). You couldn't tell which was an ad and which was the real video, so adblockers couldn't write rules to block them.
It is literally that, in fact.
A user requests a data stream, and receives it. That the user chooses not to execute or render a part of that stream is up to them - the data has already been delivered.
Interstitials, pop-ups and the like are simply contained within the stream, and aren't even guaranteed to run inside the browser. If the data connection is incomplete, modern browsers often discard data that is incomplete. Is that also illegal access?
The data stream is in the user's hands - and it is delivered before any Terms and Conditions are given.
It's similar to someone giving you a burger, saying it's free, waiting till you take the first bite, and then demanding that you eat the rest whilst they photograph you, and then use said photos to pay for making the burgers.
You could just walk away with your free burger.
They didn't save the music records, movies and video games industries. They're still actively fighting against the user, an adversary that has all the power. DVD content was successfully decrypted despite the DMCA; ironically, genuine games are often inferior to their pirated counterparts. A site without ads has better usability and is more secure.
Even if sites say you can't access content already transmitted to your computer unless you view ads, they're not in a position to enforce that. A bunch of laws saying people can't circumvent their protection won't suddenly change that fact.
They could check over time if a user has seen as many ads as page views and block them then but users would just need to delete their cookies.
I guess as blockers stop ads downloading they just need to have something like an ad or an actual one and check if it's downloaded.