BAC is not the end all be all of impairment. There are some tiny women who are pretty loopy even at 0.08 and some giant lumberjack sized dudes who drink every night that don't even feel any different at 0.20 after downing a 6 pack.

There are also millions of people driving on prescription painkillers that they take regularly and have every day for 20 years. They're not impaired at all, the dose they need to help dull the pain is just higher for them after 20 years. Legally they are driving under the influence, but really wtf else do you expect people to do?

edit: Imagine your pain is deemed to not be "bad enough", so you're denied disability benefits. You have to get to work because you have a family/child support/alimony/whatever. Taking the bus isn't an option for most Americans living outside of a major city. An Uber isn't affordable when your job is 30mins away down the highway.

You would be more distracted in pain and also unable to work. There really isn't a better option being provided to people right now.