If they were operating a "sting" using 15 officers, chances are they were not drawn from other duties, they were likely brought in on overtime specifically for THAT assignment alone. So, your thought that they would not be available for other duties would be mistaken.
Illegal fireworks cause a great deal of damage to communities throughout the state and cities and counties have a vested interest in punishing them. Sure, some of the regulations may be a bit overboard, but, the answer to that is relaxing some of the restrictions through the state and local legislatures, not breaking the law to sell illegal fireworks.
As others have stated, you are lucky. You COULD have been arrested and be facing jail time (AND the fines) rather than a simple fine.