Assuming this is a genuine offer from Microsoft, the strategy could be as simple as assuming there are going to be very many small businesses reluctant to move

Assuming this is a genuine offer from Microsoft, the strategy could be as simple as assuming there are going to be very many small businesses reluctant to move from 7 to 10 by January because of the well-documented concerns about 10, but still wanting to keep a responsible level of security updates applied on their machines. Businesses in that position seem unlikely to resort to dubious sources for their security updates. It's just not worth the risk, or frankly even the time, assuming the costs for getting the updates legitimately as quoted in the article are accurate.