Yes, it is legal. In 2013 the G7’s Financial Action Task Force issued the following statement in guidelines which can also be relevant to businesses concerned in transmitting bitcoin and different currencies, “Internet-based payment services that permit third party funding from nameless sources may also face an increased risk of [money laundering/terrorist financing].” They concluded that this may additionally “pose challenges to nations in [anti-money laundering/counter terrorist financing] regulation and supervision”.