its the public fucking highway.
the bloke parks on his drive not outside.
it only seems to be a problem in middle class/upper working class areas that people get anally retentive about people parking outside. most inner city streets people park whereever as theres just not enough parking!!!
regardless, even if some bullshit "unwritten rule" held true then its still not a valid excuse to vandalise someones car.
unwritten rules are just an excuse to make up bullshit as you go along. if they were real or reasonable then they would be written down wouldnt they??