The issue is that FCA didn't properly consider the human aspect when it came to designing and approving the shifter.
Here's the issue: the shifter that is being discussed here is not user intuitive and runs counter to practically every other shifter design on the market. In order to send the vehicle into park, the driver must push the monoshifter forwards until it selects park.
There is no feedback to the driver except on a display to tell the driver that the gear selector has been pushed into park, and it is very easy for someone who's in a hurry to put the vehicle into neutral or reverse instead of park if they didn't push the monoshifter forwards long enough.
That's fine, but the reason why FCA has been singled out by the NHTSA and is now conducting a recall is because unlike every other manufacturer that is using the same transmission and shifter design, FCA did not implement a fail safe to ensure that in the event of a failure to select park, and the occupant of the vehicle gets out or turns off the engine, the vehicle does not roll away. Simply having the vehicle scream warnings at the driver is NOT a fail safe; there are tons of stories of people ignoring various warning lights or noises in cars regardless.
I spent some time when I was in college as part of my graduation requirements in an engineering ethics class ( it was the only ethics course that had space during that semester). From the start, the prof reminded everyone that when designing something that has the potential to cause harm to people or property, in the event of a specific type of failure, the device should respond or results in a way that will cause no harm, or at least minimizes harm, to other property or to people. For example, an elevator design should automatically apply emergency brakes if the elevator cable snaps. A train equipped with an air brake should apply the brakes in the event of a leak in the brake system. It seems that the FCA engineers that implemented the monoshifter forgot this lesson.