Couple points here; 1 - The fine is assessed after the fact. The form allows the person to admit to the administrative violation, and agree to pay the fine within a few weeks. Or - one can ask for an administrative hearing by not checking the 'agree to pay' box. There's no downside to not checking the 'agree to pay' because you can always later admit to the violation, pay the fine and be done with it. Or - one can go to the admin hearing, have your case heard, and ask that the fine be reduced, or eliminated by apologizing and grovelling, etc. Of course, the fine may be imposed or even increased.
2 - The US(and AU, and NZ, etc) are huge producers of ag commodities. The San Joaquin, Imperial, and central valleys of CA are the richest production per pound on the Earth. Even a small infestation of a foreign agent can cost billions. In the late 50s early 60s the entire world production of Bananas was imperiled by an infestation. It could happen again. As noted already, we are in a crisis with bees right now, and the news is not good for many plants which rely on pollination.
3 - I'm not a big fan of a lot of admin rules, but really, for all that is holy, just check the effing box, and be done with it. Anytime I go through customs, I read the form. I fill out the form. I check the form again, then I go through my stuff mentally and verify. If I make a mistake and it's found out later, I agree with the agent, apologize, let them know it was an oversight, and if they fine me anyway, I don't check the 'agree to pay fine' box. Consult an atty, go to the hearing, ask for forgiveness, and take my chances.
I don't have a lot of pity for either the woman, or for the CBP agent. The one good thing to come out of this is it will reinforce to some small segment of the travelling public that reads this to follow directions.