You have it the wrong way round - it is not the leader telling you that you are happy it is you telling the leader and showing your appreciation for their leadership by giving back to the community.
There is plenty of evidence in history of people working for free on some project or other for the greater glory of leader, country, or the divine.
Of course there is also plenty of evidence of leaders making people work for free on some project by making it a national duty eg parts of what we refer to as The Great Wall.
So what is it that you want to change - culture boost instead of commerce boost... humm. Not sure where that is done. The trigger is in the civics infos file but I don't know that file well. You could look at the Modiki, in the menu on every Civfanatics page and see what options are available in the civics XML.
Modiki -> Civ IV XML Reference -> 3.8 Assets/XML/GameInfo -> CivicsInfos
I can't see anything in there but maybe one of the other files in that folder.