In US it is illegal to fuse the neutral line per the NEC. But individual products or appliances like this are governed by the UL or a similar agency, where each product is reviewed and tested in a laboratory to meet safety requirements.
In modern Christmas lights, both L1 (hot) and L2 are both spaced an equal distance from each other and the surface, so the danger of having exposed conductors is not as severe as it would be in a lamp holder like an Edison socket.
It should also be noted that a danger can exist by being exposed to the neutral conductor also.