This guy has paid for his copy of WinRAR

Nah, they aren't mutually exclusive.

Yeah, I agree. It is art, but if the owners sue, its vandalism.

Private property is one of the foundations of market economy, and I can't think of any necessity of art that calls for suspension of it.

On the same vein, the Mayor can't sue on behalf of private owners. If the owners like it, then its OK.

This is a criminal matter, not a civil matter. It's just like if a guy punches me on the street, but I decide it was cool and edgy and added to the gritty vibe of the neighborhood. The prosecutor can still bring charges.

You are right, my mistake.

Although it seems hard to press charges without the property owner's cooperation.

Can express permission to paint a graffiti given afterward?

Surely not. Better question: is "vandalism" without criminal trespass or property damage actually a crime? If it's on private property, and the property owner enjoys the painting, it seems like it'd be quite a stretch for the state to establish that the artist damaged the property by creating it.

You could always lie and say you gave him permission, but that would probably be a crime in itself.