Except for the open container, most states allow you to legally drive with a BAC below a certain cutoff.

Not exactly. Most states, 0.08 or above is presumed guilty of DUI without any other impairment detected. You can still get a DUI with a BAC below 0.08 but it becomes a bit harder to convict since it becomes a function of the officer’s testimony and any physical tests administered before your arrest.

Right, it becomes a matter of impairment, not blood alcohol level.

But you can certainly have one beer on an empty stomach, have a BAC of 0.04 and drive around. As long as you aren't visibly impaired, it's legal.

True true - but to drink and drive, in the most present tense form of the word drink, only in Mississippi!