Problem is you don’t know what’s reasonable and what’s not. It’s not based on how it looks. If may look reasonable and harmless but still send your IP and browser fingerprint to surveillance ads networks.
Ad blocking via DNS is relatively easy right now because a content provider like CNN.com will use a domain like “ads.evil-surveillance-media.com” to load their ads into your browser. But what happens if all these companies switch to just using their own domain to load ads? If the ads as well as the content BOTH come from CNN.com then there will be no easy way to filter the ads out. This will be the next stage in this war between ads and adblockers.