Thanks to all for the advices. I (as my websites are) am situated in France.

I agree with Wige and think that basically my "error" is not to have limited the output size of an article in the RSS feed and to syndicate all as is. I will correct that.

There are some MadeForAdsense websites based only on syndicating RSS and indeed - the fact of not controlling your RSS distribution leads to the situation that (when your site ranks naturally and well) you create yourself an unwanted competition in an easy way.

As Alienpest says it, it can be good it the relaying/copying site has good ranking.

In my precise case, it is a stupid story - I wrote a news about ethnic conflicts and the article in question got relayed on a kind of gentle nazi site. Result - I've got 90% bounce rate on this link and plenty of nazi reading my website. And I hate nazis