Thanks for that particular link, it highlights possible explanation - "Never subestimate the power of enterprise licenses". So it's entirely possible that almost no user buys it, but they get their money from enterprises. I'd love to see a breakdown if one's available.
Somebody in the thread says he bought it. My experience is that a lot of SMEs and professionals did buy it. The price was so low, and at one point its features were really essential.
Revenues of WinZip Computing (WinZip)
2003 - $25,259,000
2004 - $24,928,000
2005 - $22,700,000
Peanuts?