When you ask if graffiti is a crime it brings up a interesting question, What is public space? If it truly is public then we should be able to do what ever we would like. Now just because its public doesn't mean there are not any laws, For example you cant run around naked at a public park. That is common sense, Obviously there is a law against public obscenity, but there are no laws against art in a private domain so why would you bring laws against art into the public domain. Some people say that it defaces the property and makes it ugly to look at, but you cannot govern something based on opinion. Maybe my definition of beauty is different than yours. This is where the confusion occurs because you have found yourself in a paradox. To me there is no answer to this question everyone has differing opinions on the variables. Rather than should it be legal we should be asking ourselves, What is public space?, How do we govern opinions, and What is art?