Performing in the streets sure is legal but graffiti isn't. For this to have a high traffic, the graffiti has to be done in a public place where a lot of people can see it. The artist of this probably either owns that wall or had the permission to paint there. There's no way he could have painted that in minutes and not be able to be found by the police.

I don't think he actually made the graffiti for the money. Obviously there will not be a lot of money coming to him because not much are using bitcoins. I think he made that graffiti because he knows that it will be viral and will be talked about in forums like this. He knows that it will raise more awareness to the people and it will reach even the people who weren't physically there like us. And perhaps the QR code was just placed there so when someone might wanna tip. Leaving the QR Code is really smart since he can receive the money without having to be physically there.

Unfortunately for him, his QR Code can easily be painted over by someone as well as the painting overall. It would be nice if someone can translate what's written on the upper-left part as well as what's written on the paper money the guy is holding. It says "debt" tho maybe he's trying to say that we're all doomed to have debts in the fiat monetary system.