For Frenchie, the moose symbolizes the natural world that's unchanged, watching the world change around it. Taking down the moose would be taking down any hope of restoring that natural world and the history stored in the memories of both the moose and the land itself, which bears the scars of the trauma that's been inflicted upon it. Choosing to let it live shows that Frenchie is choosing to not act like the government that's after his marrow.