Sadly, laws tend to cater to the “lowest common denominator”. There are folks who are quite “impaired” at this lower BAC. While they are generally in the minority, they do exist (I’ve known folks who could have half a beer and you really didn’t want them driving for a while... scary).
For the majority, this lower BAC is probably ridiculous. BUT - lets think about it another way: If you are a responsible driver, who HONESTLY is not impaired at .05%, and you obey the traffic laws while driving - this won’t be an issue. Those who ARE impaired at 0.05% - the law will give bigger teeth to.
In Mississippi, the last state standing, it is still legal for me to stop and buy a beer for my drive home from work.