One of the definitions of hack is "cut with rough or heavy blows".

It's on Hacker News not because of the tree itself. It's on Hacker News because the Internet coverage of the tree is what made it notable enough to vandalize. Many of us build the tools of an online society. We live largely within that online society, but not exclusively.

Why wouldn't we care about the acts carried out offline being influenced by our activities online?

Why do people insist on using this line to criticize the relevance of submissions to the community? You're presumably arguing submissions like this degrade the quality of discussion, but won't submit any reason it isn't relevant to the board. If all you're looking to do is shit in the punchbowl there's other boards for that, and if you think it's completely and blatantly obvious the submission is unsuitable there's a "flag" button available to call a discussion to the attention of the moderators.

My 2 cents, I believe it belongs here because the question of whether social media is a net benefit to society is frequently debated here, and this article uses the tree as an example to argue that it is a net negative. There's some interesting discussion in this thread about attention-seeking vandalism in the context of the internet.

Because someone submitted it.

Why not? If it wasn't interesting people wouldn't have up voted it. We each have our own interests.

Before clicking I expected the link to be about a data structure.