The problem is that it is a war - between advertisers and those people who are trying to block ads. Companies like ours and AdTrap block, they find a way past it, we block their new method, cycle repeats. Yahoo makes most of their money from ads - they are not going to make it easy to be blocked. Adult sites (for example) are the reverse - they have an interest in being properly categorized (and blocked) so they keep out of the spotlight and can survive in self-regulation.
There are two core methods of blocking: - Blocking all content that comes from a particular source - Advertisers get around this by either making the source dynamic or by mixing real content and advertising from the same source, which means if we block we are getting false positives - Blocking based on behavior (eg blocking pop-ups and banners) - Works well but can have a high false positive. By blocking banner ads you sometimes don't see real buttons.
I personally use NoScript for FireFox, which uses the second method. It basically blocks by default and when a page doesn't work you have to allow certain content.