When they tried this with parents who were late picking up their children from daycare, it actually made them later - since now as long as they could afford it they didn't feel bad about paying.
Similarly, maybe those Tesla owners won't mind paying $0.40/minute for convenient parking.
Kind of sounds like they had this in the pipeline, and rolled it out after the tweet. But hey, the tweet is a good way to turn this into a "Elon Musk is great at customer service" story rather than a "Tesla suddenly rolls out new fee" one.