Answer:
Tolerance to alcohol does not affect immediate measures of BAC. A tolerant person can have a much higher BAC without feeling the effects but according to the law they are more drunk than a person with a lower amount that is highly affected by alcohol.
To be more specific, high tolerance essentially affects the level of habituation a person's neurons have adapted to alcohol and reduces the number of receptors that they have for alcohol. As BAC is a measure of immediate blood alcohol levels, this is not affected by levels of receptors in the brain.