There is a big difference, actually. There is pretty standard protocol and work agreements for automotive repair. This developer may have exposed himself to a level of risk based on his own contracts and work arrangement. Why was the site up in the first place if he was not paid in full?
"Similarly, I've met software developers who set timebombs in custom software they write for companies, with an easy to install patch that gets released when that company pays it's bill."
Wow, this is just scary. The work relationship is doomed if the developer doesn't trust the customer or vice versa. If you are in the business of making money through software development, be skilled not only in developing but in assembling contracts and project plans that protect your interests.