I was curious, so I just checked the FMVSS requirements for these[0]:

Must be activated when the ignition switch is energized and reverse gear is engaged. Must not be energized when the vehicle is in forward motion.

Seems that should be amended to not allow use when the vehicle is in park, just as they are prohibited while in drive. I'm tempted to write to the NHTSA and propose this change.

[0]: https://www.ecfr.gov/current/title-49/subtitle-B/chapter-V/p...