That's a misinterpretation of DUI laws. Even when the legal limit was .10, it was still possible to be found guilty of DUI if the state could prove impairment. In general, no one under the limit would be charged because the state would have to prove that the driver was truly impaired, rather than present a blood test.
The legal limit is simply a level where impairment is legally presumed.
One class of wine would leave a 100lb woman with a BAC of about .04.
Even when the legal limit was .10, it was still possible to be found guilty of DUI while under the limit, if the state could prove impairment.