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?
Well zoning in on the rare cases is the difference between what we (as in society's collective technology, not tesla) have today and full reliable self-driving.
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.
