Curved mirrors also take getting used to, but I doubt it has ever been shown that the number of errors people make because of using curved mirrors is larger than the number of errors people make while using regular mirrors.
These kinds of rules get introduced because someone thinks "this makes sense", other people reading it think "this makes sense" and nobody with enough actual knowledge about the subject, who cares enough and has enough lobbying power, goes "now wait a second, this is not right".
I don't condemn anyone for taking decisions based on what seems to make sense, when the stakes aren't very high and in the absence of anyone telling them otherwise. We can't expect decisionmakers to be experts in everything and we can't expect them to hire experts for every little detail. As long as things get changed when the facts are presented, I'm not complaining.
I'm even more interested in the number of errors people make from curved mirrors compared to the number of errors people make from having no mirrors at all (regarding the blind spot area)