You’re from Pennsylvania, right?
Our DUI law permits a finding of guilt for (a) BAC > .08 or (b) impairment in the opinion of the officer (weaving, field sobriety test failure, etc.).
I guess there's some logic in that. As others have stated, other drugs could impair a driver without a BAC. That said, I'd have a hard time finding someone guilty who had a BAC of less than the legal limit and no other drugs in his/her system.