I don't think there is much to stand on here. Paying up front for the suppressor is our choice and forced by the businesses that sell them. If people were unwilling to pay until the approval went through, it would force business to change their models if they wanted to continue to receive the revenue. The government doesn't physically hold the hardware.
I'm not saying it IS a due process issue. Just playing along with the hypothesizing.
It's bc this is gun related that it's allowed to stand. Not enough outrage from the masses, only "gun nuts." If the government had an approval process for home buying that involved passing an inspection before being able to occupy the house that took 6-9 months after settlement you think that would fly?
I may be comparing apples and monkeys here, but I'm just saying. We all know the wait times are outrageous.