My point is not who is right or who is wrong. On the one hand, emotionally, one can see that he did (and does) something beautiful, and he didn't damage the "public property" in any way; it is definitely an improvement over graffiti or just empty concrete. On the other hand, yes, he did commit an act of "unsanctioned painting and writing on public property" without getting permission from "the public".
Incidentally, "the public" in such cases tends to be some local bureaucrat, who, for the most part, was never even elected by the people. Nor can he be replaced by the people; he might get replaced through an elected official, but only if he does something truly terrible. So, in what sense these bureaucrats represent the will of people is not completely clear to me. See more about the rule of civil service here. In particular, this video:
What my point is, however, is that had this been private property, things would be much simpler:
1) Either the owner would agree for the artist to paint on his property or he would not. This way, there would be much less legal, moral, emotional, utilitarian, and "rights-ethics" ambiguity.
2) A private owner (while interested in protecting his property from random vandalism, graffiti, obscene writing and pictures) would probably be quite happy to have such a painting as the above on his dock, since not only would it be aesthetically pleasing to him and the public, but it would also make his dock more attractive and more famous thus increasing his profit.
(On the other hand a bureaucrat does not care about making what he is in charge of profitable. If his budget — assuming he has one — is unbalanced, or the "public property" he is in charge of is losing money, he can always get more money from taxes or government printing/borrowing money.
And obviously he will say that he needs more money, since what he is charge of is benefited from by "everyone", even if it is a dock in Volgograd, while he is taking money from a restaurant owner in Moscow. With private business it is much simpler. If the restaurant owner from Moscow makes use of the dock in Volgograd, the dock owner can send him an invoice.)
The artist's rendition of government and bureaucratic civil service: