Unfortunately that's the way the law works. If you write on some property that isn't yours, it doesn't matter if it disappears in a second or lasts forever, you

Unfortunately that's the way the law works. If you write on some property that isn't yours, it doesn't matter if it disappears in a second or lasts forever, you've still written on some property that isn't yours and you've committed vandalism in California. The restriction to being able to exercise his freedom of speech is another issue entirely.