You don't say why PTA says no, but I'm going to guess that it is for insurance reasons. There are basically a couple of agencies that handle the policies for PTAs/PTOs/booster clubs, etc. For one of them, in their PTA coverage summary, sales of fireworks are specifically mentioned as a hazardous activity, and you are told not to do them. If you do, you could lose coverage. But, damages from fireworks sales are not specifically excluded.
It's worse for PTOs--in their coverage guide for PTOs, damages from fireworks sales are specifically excluded. So, you can do them, but you're not going to have any insurance coverage. Someone gets hurt from fireworks you sell, and they sue you, you're on your own.
I am currently researching PTA vs. PTO pros and cons. One of the things our current PTA would like to do is a Fireworks Booth during the summer. Pta says no...anyone know if it is possible as a PTO?