From what i've gathered on the interwebs they couldn't prove that Rossi deliberately kicked Marquez off the bike.
Maq said he kicked him and hit his handlebar/brake, that this caused the front to lock up.
Rossi claims his foot slipped after they touched.
Everyone saw Marquez' version of what happened. I'm fairly sure the stewards would have been able to prove that it was deliberate with pretty much any driver apart from Rossi
The penalty points he got were for pushing Marquez wide.
The stewards even claim that Marquez was just as much at fault as Rossi, even though he drove well within what is allowed by the regulations. So they judged Marquez intent to f*** with Rossi and feel bad they can't do sh*t because there's no rule by which they could penalize him, but when the video material from all cameras clearly shows that Rossi in some form kicked Marquez it's not 'clear enough' to penalize him for that because Rossi said the opposite...