That was my thought too. black and silver aros are the same to me except for their scientific name and coloration.I am not aware of anyone in our circles who understands why Australian, Asian, African, or SA black arowana are any different with respect to a potential effect on the Florida ecosystems than the SA silver. I get your question. Black and silver are like GMC and Chevrolet, or six one way and half a dozen the other.
I have never met an explanation. Not that I looked anyway. The government doesn't have to explain but I am sure if one digs up the legislation papers, something could be understood. I for one have better things to do as doing something like this sounds as exciting as a short and senseless jail sentence to me personally.
I even think the ban on asian aros is ridiculous. Since 99.9% of all asians on the market is farmbreed and has to come with legit papers and a microchip just like any purebred dog or cat. But thats for another discussion IMO.