Google has removed ad-blocking Android apps like Adblock Plus from Google Play because these apps are breaking one of their rules. Section 4.4 of the Play store developer distribution agreement says:
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.
Android users don't have to stop using them. You can still sideload apps (without rooting) by allowing installation of apps from "Unknown Sources" in the settings. Adblock Plus blog says that the next release will have automatic updates that are independent from the Google Play store so updating manually will no longer be necessary.
Please keep this in mind that free apps and websites like Winmatrix.com are still alive because of ads. You can always exclude your favorite services from Adblock Plus. There is no ad at all on the forums for the registered users of Winmatrix (registration is free) but if you don't want to register, you can still contribute by excluding us from your Adblock extension.
Source: Android Central