It can be dangerous as well as misleading! When I visited relatives once, my aunt asked me to take a look at her daughter's computer because it was acting "funny". A bit of investigation found a massive collection of viruses, malware, and spyware (around 250); after it was all cleaned up, things seemed back to normal. My cousin was quite surprised it was infected with anything -- because she had been running a virus scanner every time a pop-up window told her there was a problem and she should download something to fix it. After that, ALL the family members were given a tutorial on "never click on advertisements no matter what they tell you" and "don't trust anything except the virus software you already own."

Also, I don't think AdBlock is stealing any more than throwing away the advertising section of a Sunday newspaper is stealing, or walking out of the room while a television commercial is playing. I have ever right to restrict what information arrives at my home -- or is it also unethical for me to block my children from porn and other adult content?