Because there couldn't be any legitimate reason to do the things they're banning, like cloud seeding, crop dusting, air dropping seeds for reforesting, I dunno, literally releasing anything as you fly over even like CO2 exhaust as mentioned by the other commentor.
Literally all matter is a chemical, chemical compound, or substance. IMO this law is going to be struck down super fast just for being overly broad. Not that that would stop Republicans from passing it and spending millions of dollars in public money defending it in court.
The author assumes the Court doesn't understand the consequences of what it's doing, but I really don't think that's a reasonable assumption. It's entirely possible they know exactly what they're doing.