Cannabis is a drug that can serious addiction, it is used under medical control and is not banned just that it is a prescriptive drug. Rose on the other hand is a flower. IMHO it is quite obvious, nothing to do with culture.
All this news is really making me feel bad for the Saudi people. I personally wouldn't blame this on Islam because given the choice, I'm sure a lot of Saudis would really celebrate V day... it's just the goddamn government that's responsible for all this bullshit.
Roses are very addictive and as dangerous as cannabis. Try to give your girl friend roses only once...