True, but he's a conscientious guy trying to do the right thing. Going to the town specifically to go to the police station seems better then walking in and saying "Oh, by the way, I saw a severed arm on the road... I don't exactly know where, since I'm a hitch-hiker, but it's back 14 miles down that road, and no, I don't know if anything will have savaged it, possibly destroying evidence, by now."
And it's not IN his backpack, it's hanging off the back of it, which is at least easier to clean, I suppose.