When an entire community / culture of art is built around vandalism, then yes it is excusable. Graffiti should not be illegal. Artists struggle with a lack of resources, a lack of exhibition space, and lack of community engagement. Street artists bring art to places that are missing the opportunities to view it in institutionalized museums, and the artists get to show their work. To understand why art-based vandalism is important you have to engage in the artists reasoning, and the thought process behind the art.