Google's ads are click-based. Users who block ads aren't the users who click ads
It encourages advertisers to be more judicious with their ads. If their ads aren't obnoxious, they won't get blocked.
Google doesn't want to open up the vault that is banning extensions. If an extension is safe and does exactly what it says it does, it can be hosted on Google Apps. Not following that model is a really easy way to piss off the userbase.