I see plenty of ads in my feeds. And nothing to stop Google from placing ads in Reader itself.
1 ad per week on a RSS feed vs 1 ad per week on an entire website
and the answer would still turn out to be Ad Supported Websites. RSS users are an insanely small group compared to overall users on the web. So yeah, Google probably just decided that supporting RSS just isn't worth their money/time regardless of how it compares to its other services.