After thinking about it a little more, I agree to an extent with what you say. In certain cases, ad-blocking is acceptable (though probably not sustainable if enough people do it, and said sites lose money as a result). I've done some research and found that some sites, in their terms of service, prohibit the use of ad-blocking software, so one would have to constantly be on the lookout for sites that present their content on such terms. In those cases, I would definitely consider it to be an immoral action – violating a contractual agreement.