Joel would say that, since "incentive pay" comes out of his profit!
The prospect of more money wouldn't convince me to do a job I actively disliked but at the end of the day I'm a professional. I do what I do for money. The company isn't shy about boosting its revenue and making profits, why should I be?
Joel would be one of the least concerned about how incentive pay affects his profit - he spends way more than average per developer, but in the areas where he thinks it has a positive effect, like above average salary, good desks/chair, etc.
You may dislike Joel, but but that's a stupid argument.
I like Joel, and have subscribed to his blog for years, and what he say often seems to make a lot of sense. However, I know a lot of people in this industry, and I don't know a single one who uses a Fog Creek product. That, to me, speaks volumes.