Ask HN: Why does Google adblocking extensions?

Since google generates revenue from ads aren't they losing money from then not being displayed?

Examples: Adblock plus: adblockplus.org Adblock: getadblock.com Adblock for youtube: https://chrome.google.com/webstore/detail/adblock-for-youtube/cmedhionkhpnakcndndgjdbohmhepckk/details?hl=en-US