Not all unofficial communication is illegal. However, it is the ubiquitous illegal urban graffiti that gets the most attention.
Every day people run red lights, write bad checks, commit arson, robbery, and assault--far more dangerous crimes than writing graffiti. The visibility of graffiti is proof of our fragile hold on "order." We can see the accumulation of illegal pictures and names on public surfaces, wheras most other crimes occur in relative obscurity.
One of the main reasons graffiti persists even though the penalties get harsher is the power of the idea that "THEY (the police, the authorities, the government, the public) can't stop us."
The illegal writers remind the rest of us that the effectiveness of laws is dependent on people's willingness to obey them. Graffiti persists because there are more writers than there are police officers to stop them.