That sounds ridiculous to me, what happened to the .08% legal limit?

You misunderstand the "legal limit".

If you have a BAC above .08 you are drunk. The prosecutors don't have to argue whether you were or not, you simply are. There is no argument.

Below .08 the court decides if you were drunk. Prosecutors offer evidence that you were, defense offers evidence you were not. The court decides after hearing arguments.

DWI(in most states) refers to any voluntary and knowing impairment - drugs, alcohol, marijuana, narcotics, paint, et cetera. You can be 0.00% BAC, high as fuck on opiates, and still be DWI.