It is theft. Just because the courts don't know exactly what the heck is up with the internet, doesn't mean it isn't illegal. It's already an actionable offense when it comes to
lawsuits.
How is it not stealing? You are artificially taking commission from other affiliates or advertisers when THEY do the work of selling the product. In what world do you live in that what you are doing is not stealing? Just because you hide behind your computer screen and are semi-anonymous on the web, doesn't mean stealing from people isn't wrong.
edit- looks like erect beat me to the punch.