Automatically implement AdSense section targeting to improve the relevancy of your AdSense ads.
Nope, this is not against the Google Adsense Terms of Service. This is what Google says about it:
"You can use section targeting to make suggestions about as many sections of a page as you like. However, please be aware that we can't guarantee results, and that it may take up to 2 weeks before our crawlers take into account any changes you've made to your site.
In order to properly implement this feature, you'll need to include a significant amount of content within the section targeting tags. Including insufficient content may result in less relevant ads or PSAs. In addition, please keep in mind that this feature should only be used to emphasize significant sections of your site's relevant content. It is against our program policies to manipulate the ad targeting to result in ads that are not relevant to the content of your pages." Google AdSense Help