According to multiple sources, Google has started sending notifications to developers of ad-blocking software (Eg. AdBlock, AdAway, AdFree, etc.) notifying them that their applications are being removed from the store. Google says that these applications violate section 4.4 of the Play Store developer agreement:
You agree that you will not engage in any activity with the Market, including the development or distribution of Products, that interferes with, disrupts, damages, or accesses in an unauthorized manner the devices, servers, networks, or other properties or services of any third party including, but not limited to, Android users, Google or any mobile network operator. You may not use customer information obtained from the Market to sell or distribute Products outside of the Market.Now, how does ad-blocking software tie into this? It comes down to the portion regarding third-party software, which ad-blocking apps clearly interfere with. It doesn't help matters much that Google has a large interest in this industry, seeing that they run AdSense, which makes up the majority of their revenue. However, this only affects the distribution of these apps on the Play Store, so the developers can setup download links for users elsewhere.