I feel mathematical right now. Don't know why.
1 game here in the US costs roughly $63.
The fine is $1,000.
That is practically about 16 games sold to cover that one fine.
Using the Acti-Blizz guy's numbers: 5,000,000 x $63 = $315,000,000 dollars. Now let's say 10% of that 5 million was sold to minors because of visuals or such, not because the seller knew the child was underage. That'd be 500,000 games.
Cost of fines: 500,000 x $1,000 = $500,000,000.
$315,000,000 - $500,000,000 = -$185,000,000.
That is a negative cost. And since the Publishers really only get about 10% of that $63 profit, the publishers would easily go out of business trying to pay all the fines. That is excluding retailers and developers.
Math doesn't lie sadly. Plus, you have to take into account the people that say 'They never told me it was a 18+ game! They're legally binded to do so!' Or those that buy the games for the kids as gifts, or those that bought the game full on knowing that it was 18+, but down the line say that the child bought the game themselves.