What are you going to do? Turn the inside of a car into a sterile wasteland and ban every possible thing that might decrease someone's ability to drive by event the smallest amount? Hey, no radio, phone, GPS, and definitely no talking to the driver. No driving hungry, or after taking cold medication, or after a Red Bull. All of those things could impact your driving in some minor way.
It's a question of proportionality. There is a point of diminishing returns beyond which the effort required to prevent people from driving after drinking becomes absurd. We can't even successfully prevent all idiots from driving at .08, despite millions in enforcement and PR campaigns. Imagine the pointlessness of spending an order of magnitude more to also fail to stop people from having a beer with dinner.
There is a point at which alchol impairs your ability to drive a car to the extent that you are an unacceptable danger. That point may be .08 or it may be .05, but it's definitely not "anything above 0".