It could also be the lessening influence of Microsoft on the software industry. A decade ago, big companies were used to paying per seat because the bedrock apps like Word and Visual Studio were always licensed per seat or per server. With the rise of smaller competitors offering online tools and software either for free or with site-wide licenses, big businesses might try to avoid the hassle of tracking every user added and subtracted.

(It's also possible that Microsoft's pricing model has less influence on Joel since he's been away from the company for a long time, but that's just speculation.)