Graffiti typically means "illegal", so that's why the headline sounds outrageous. 5 Pointz was more of a "graffiti museum", with substantial cultural significance. It was a nice piece of news to see for a change. I was heartbroken when they whitewashed that building.
In this case, the owner rented out studios to artists for years, and permitted/encouraged them to paint on the walls. He also designated different walls as "permanent" and "temporary" spaces. All that points to a contractual relationship between the owner and the artists. Remember: contracts don't need to be writing.
It's also not entirely clear if the property owner was ever the owner of the art itself. There's a difference between posession and ownership, and while the former is 90% of the latter, that leaves 10% up for grabs. From this pov, the case is maybe comparable to someone finding a Picasso that someone forgot on their front yard, and immediately destroying it.
But, in general: yes, if your property somehow becomes valuable for society in some way or another, that may entail new restrictions on your use. Landmark and similar designations for historically valuable buildings come to mind. When some endangered species starts breeding on your property, you may no longer be able to turn it into a parking lot.
Property rights just aren't as absolute as people like to think, nor are they not subject to change. It's obviously in society's interest to provide for robust protections of private property because it creates incentives to own stuff, and to be productive in order to do so. But these rights are just a tool that has proven extremely useful to organise our economic activity. There are always competing interests, and societies have to find a balance that maximises total value while remaining fair to each individual.
The same principles would be at play if you were letting a friend store their bike in your garage on weekdays. One tuesday in April you call them to say "I need to paint the garage in June, come and get your bike". But they're out of the country for the week, so the next day you throw away the bike.
You would have the legal right to clear out your garage, you had no signed contract to rent out a space for your friend's bike, but the way in which you did it caused your friend to lose their bike after you had given them a reasonable expectation that they wouldn't, and you had no good reason not to wait until the end of the week.
That's for a judge to say. My brain's ethical lobe suggests that as long as the friend invested less effort in getting the bike out of your garage than you invested keeping it in your garage, it's fine to throw it away.
If you give people the reasonable expectation that pictures drawn on your property will not be erased in the near future, I don't think it's unusual for you to be held to that expectation. Not all liabilities are written on paper with a signature at the bottom.
