The reason behind it: it is a well known strong carcinogen.
So are cigarettes and the like but those are easy to get. I find such regulations very dubious to say the least, after all, who would order stuff like that if they didn't have a practical use for it and know what they are doing? I don't think people with no idea of what a Gum Print is would ever order Ammonium Dichormate.
Sure, I could give you a boring explanation who I really am but I rather let the Origami do the talking.