I guess you could think of spliffs as an allegory for everything that should be verboten, especially in public. I guess there are two levels of analysis here.
A) He may indeed be more rebellious than his older brother as he never felt a part of the bubble. Predominantly, because he is sixth in line, so why would he care anyway.
B) It could also be the image sold by the media. I am not coining a conspiracy theory here, but how media portray somebody matters a great deal. Perhaps, William also had a few bloopers in the past, but it was toned down and Harry's adventures were not. Such a contrast works in favour of the Royal family. All the loyal subjects of the Crown will be able to sympathise/ identify with one of the two images. One being strength, stability and responsibility represented by the older brother and the second one being a closer to people side of the family with a human face.
Also, you always need a black sheep in the family to make the other sheep look better.
