He's explaining to her:
"They cross the border into the forbidden zone and pick them. They flourish there.
Only, you see, they can't sell them - they're still far too radioactive. So what they do is put maybe one, two, three berries in each pint. They average them out so there will be no harm. One berry's just a little more radioactive, say, than a truckload of bananas...it's THERAPEUTIC!"