They know God is in their midst. The Children of Israel are stunned. It's easy to be good when you don't have to, because there's no obligation to make you feel trapped and resentful. But when you're expected to clear the table, it gets your back up, and then being good is an altogether different and greater challenge. Goodness that comes and goes on a whim is neither meaningful nor reliable.
Real goodness is accepted as an obligation.
Autonomy from constraint is a core American value. Pilgrims seeking religious freedom settled the 13 original colonies, and flight from political and religious coercion continues to fuel immigration to the United States. A father needs to come home and feed his kids every night, even though he doesn't always feel personally rewarded. If each person's priority is his own fulfillment, you can't count on anyone. Nadav and Avihu didn't just value autonomy.