Why would they check if Adblock is installed or not?

https://code.google.com/p/chromium/codesearch#chromium/src/c...

  bool ChromeContentRendererClient::IsAdblockInstalled() {
    return g_current_client->extension_dispatcher_->extensions()->Contains(
        "gighmmpiobklfepjocnamgkkbiglidom");
  }

  bool ChromeContentRendererClient::IsAdblockPlusInstalled() {
    return g_current_client->extension_dispatcher_->extensions()->Contains(
        "cfhdojbkjhnklbpkdaibdccddilifddb");
  }