Even if a substance like this was made illegal to purchase, especially in commercial sport stores, I don't think it would stop someone genuinely looking to cause a deadly explosion in an urban area like New York City.
Lets pretend that this particular substance was banned,and it was "impossible" to obtain without a license or whatever. If it is a compound of two non-explosive materials, can't the individual looking to cause harm with this just purchase those two components repeatedly, and mix it themselves? To crack down on such specific substances like this is difficult, as you never really expect people to do these things until they happen, and that that point it is too late