Maybe beached whales are considered the state’s property, so cutting its tail off would be theft from the state? I can’t think of any other reason this would be illegal.
But, really, arresting the guy? That’s a bit much. I’d hardly consider him a dangerous crook for making a mistake any beachcomber could make.
Many rules are like that to prevent poachers from claiming “it was dead when I got here” and forcing the state to prove that the person cutting apart the carcass actually killed it. Instead the state just cites the poacher for taking the meat.