I believe it was relatively recently and the rationale was two fold for the time penalties. Firstly is provided an option for a penalty that was less than a stop go or drive through. It is also more consistent across circuits, as a drive through could be a really small penalty at a circuit like Silverstone. So you might give a drive through or stop go and the impact could vary by many seconds depending on circuit. I suppose also it clarified, or at least made it more consistent, about what you do if a penalty is given out near the end.
Edit: 2014?
https://sidepodcast.com/post/permane...oints-cost-cap