Guess you weren't on the receiving end of the certified letters from WU's attorneys about using their trademark material from the web site then. It may have started out in the free domain courtesy of the NWS but it is being put out on the web by WU using their $$ and resources. I just think I'd make sure before just assuming it is free for the taking.
From WU: Upload, post, email, transmit, link to or otherwise make available any information on the Site that infringes any intellectual property rights or other proprietary rights of others, or that contains any virus, computer code, file, or other material which has as its intended purpose, the interruption, destruction, or loss of functionality of any computer software, Data, hardware, telecommunications equipment, or communications ability;