In the past six months, [spyware] infections prompted 850,000 U.S. households to replace their computers.
I imagine that most of the new machines run Windows. Microsoft's near-monopoly position may mean that the company brings in more revenue by allowing spyware to thrive. Never mind the fact that the cost of designing secure code and safe user interfaces would eat into any company's profits due to software insecurity costs being an externality. Hampering spyware could hurt Microsoft's revenue.