The point is also to minimize potential damages caused by a bug in the software. Just this year there have been multiple data-destroying bugs in publicly released software. If the app runs as a server it’s usually trivial to have it run as a dedicated user, with just enough permissions to do its job.
It’s just good practice, even though the risks might be low why risk it at all?
No, a bêtise is when a kid spills her milk or breaks a toy.
In this context I think they use the word as an euphemism for cheating on your spouse, as that’s the defense of most of the accused: they thought it was a kink and that Gisèle Pelicot had given her consent.