Thoreau enchained?

To turn a phrase, I suppose I could say that I detect irony like a magnety. If there's something ironic to be seen or heard, I'm likely to notice. My detector went off when I visited Walden Pond and saw this statue of Henry David Thoreau, with chain looped around leg and adjacent tree.  I'm not sure if this is an anti-theft measure, but the image's symbolism seemed distinctly odd.