Google isn't the only one who makes money from ads. All of the free websites that you know and love (including the one you're on right now) depend on ad revenue in order to offer you the free content that you've come to expect.Of course,
Google has decided that ad-blocking apps are in violation of Section 4.4 of the Android Developer Distribution Agreement, which prohibits any app "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."
It's hard to argue that ad-blocking doesn't violate that section of the agreement, but that still begs the question of why Google hadn't enforced this before now. Ad-blockers will still be available from outside sources, but they will no longer be found in the Play Store. So, be extra careful if you're going to install one, since side-loaded apps have a higher incidence of malware. Not to mention, as we said before, the sites that you know and love only exist because of ad revenue, so you should think twice before using one anyway.