Welcome to r/pics.
Not exactly the brightest bunch around here. I'm with you, though. I think people have misunderstood the intention behind your comment, perhaps, or maybe they're just being persuaded by the current vote counts (I think people often vote, especially downvote, when they see someone with many votes in either direction).
However, you are absolutely right. This is not worthy of a prison sentence. Prison is for people who are unsafe towards the public. This person is a giant fucking asshole, but he is safe towards the public. He hasn't harmed anyone and this is a first offense. Why should we have to pay more in tax dollars just to feel good about ourselves? Stupid. There are many other ways in which a person can be punished and which do not subtract any further from society.
The only reason why people are wanting this person in Prison are for purely emotional and illogical reasons. He heavily damaged one painting*. It is not as if he is going around and destroying many things. One painting. Just one. If he was caught doing it time and time again, then sure, he is obviously incapable of remaining in public and so then should be removed. That is not the case, whether you -want- to assume otherwise or not. If you believe he might, given that he is clearly an asshole, then maybe we should just say fuck it to the whole 'guilty until proven innocent' thing, yeah? Yeah. He does not deserve prison. And before someone says that he needs to feel sorry for his actions so he can be 'reformed', then maybe you should look up current rates in regards to that. Prison doesn't do shit to 'reform' the majority of people.
*The value of a painting is also incredibly arbitrary, unlike that of say a house or a vehicle, which have far more defined costs. Just to address those of you who are using the value of said painting as justification. It's a bit ridiculous. And you are only using it's value to justify how you feel and what you want. The vast majority of you don't actually give a fuck about the painting what-so-ever-at-all, you only do now because you want to feel self-righteous. "This was a historical piece! It is part of our culture! It's incredibly important!" etc etc etc. Horseshit. You people don't care. I doubt most of you even understand why you might care. You're just using these ideas to dishonestly justify your feelings.
TL;DR - Stop subtracting away from society just to satisfy your emotions. Punish people appropriately and effectively instead. He damaged one painting of which none of you have EVER heard about before. You are acting like he goes around and does this all the time. One offense should be met with a hefty fine and, in this case, significant community service. Multiple offenses is debate for removal from society, of which still does not reform people who stand the chance to be contributing members of society. Be sane and logical about this shit, it's kind of important as it helps set precedent down the line in other areas of debate.