So, in the text, Katniss putting flowers on Rue is a rebellious act because she gives Rue value and mourns her death. The Games are supposed to be Lord of the Flies, every Tribute for themself, alliances are OK but you're gonna have to kill them later, and basically dehumanizing the children. The Capitol made the Games to force the districts into compliance by killing their children, rather, having their children kill each other. One of these children actively mourning the death of another child goes completely against that.

Then the salute was a slap to the Capitol's face. Then the berries. It's no wonder Snow spends the next two books getting revenge on her.