The theory would be that washes out in the noise. It's a simplification, but on average, most of the people most of the time are not doing that - why would it zone in on the rare case and apply that behavior?
Even in the anecdotes throughout the rest of the comment section, there's a lot of people that said "yeah I tried FSD for a limited period of time and it worked for me". Because we're not saying that taking FSD outside right now will kill you within 5 minutes. We're saying that even if it kills you 0.01% - that's pretty sketch.
The general principle is that all of the drivers that have been recruited to be 'teachers' to the machine are not aware that they are training a machine. As a result, they are probably doing things that are not advisable to train on. This doesn't even just apply to machines - how you drive when you are teaching your teenage child is probably different than the things that you do on a regular basis as an experienced driver. If you are not aware that you actually teaching someone something, that's a dangerous set of circumstances.