Have you ever been to the Colosseum? There's areas of graffiti all over it in all sorts of forms if I remember it correctly, as I'm sure millions go through it each year and have for quite some time. It's just that these two got caught by being extra stupid and taking a selfie which would no doubt go on instagram or facebook. The punishment should fit the crime and that punishment should be a fine that causes some pain as well as possibly - a ban on traveling to Italy if they wanted it to be extra severe.
Past graffiti in the Colosseum: