Public washrooms and religious graffiti.

I've noticed a lot of public washrooms have religious graffiti in them lately. as Simon and Garfunkel once sang "The words of the prophets are written on the subway walls and tenements halls". How odd how the bible seems to preach vandalism. Or at least some of its adherents seem to believe vandalism is justified so long as it is proselytizing their religion. I couldn't find with google any admonitions against graffiti, but I did find psalm 17:7-12. "Paint grace-graffiti on the fences". Perhaps I'm missing some significant subtext into why this is considered acceptable behaviour.