I think it's always been pretty obvious that Rihanna views the world as her personal playground...
It was a childish thing to do, which might have seemed funny to her at the moment without giving the other woman's embarrassment at an oily butt with popcorn crumbs a second thought. Just childish.
I think most of us have pulled stupid pranks, but at a much younger age, like me in 5th grade when I glued a classmate to her seat. It stopped being so hilarious when she couldn't get up and the teacher was on me like a duck on a june bug, and even less funny once my parents got hold of me that afternoon, since they had to replace her pants. Lesson learned.