It's working because it's not very used. It's sort of a "pirate seagull" theory. As long a few people use it it works. Because scrappers don't really count on Anubis so they don't implement systems to surpass it.

If it were to become more common it would be really easy to implement systems that would defeat the purpose.

As of right now sites are ok because scrappers just send https requests and expect a full response. If someone wants to bypass Anubis protection they would need to take into account that they will receive a cryptographic challenge and have to solve it.

The thing is that cryptographic challenges can be very optimized. They are designed to run in a very inefficient environment as it is a browser. But if someone would take the challenge and solve it in a better environment using CUDA or something like that it would take a fraction of the energy defeating the purpose of "being so costly that it's not worth scrapping".

At this point it's only a matter of time that we start seeing scrappers like that. Specially if more and more sites start using Anubis.