You happened to have an ethical prosecutor, who has better things to do that try to ruin someone's life, after they used good judgment, and kept their alcohol intake low. When an officer arrests you, at the roadway, the quantity of evidence can be pretty low --- less that 50% likely you were intoxicated. Then at the station (in North Carolina --- you are read both Miranda and implied consent rights, which explain that you need to submit to breath testing, to save your driver's license. You submitted; the breath test showed low numbers; your charges were dismissed.

Here is a paragraph from one of my books, on drunk driving:

The rules outlining what constitutes sufficient “probable cause” or reasonable grounds for a custodial arrest for DUI-DWI vary from state to state. Whether or not there was sufficient probable cause or reasonable grounds for your arrest for DUI-DWI will depend on your state’s case law, statutes and constitution, plus the circumstances surrounding your arrest. Having the smell of alcohol on you alone is typically not enough “probable cause” for a custodial arrest for impaired driving, unless you are under age 21 and alcohol is a prohibited substance for you to possess. In Texas, where no “open container” law exists, just because the police see the driver with an open bottle of beer in his hand, it is not by itself a sufficient reason to stop you. This is why your DUI-DWI specialist attorney will need to know all of the minor details of your arrest. These details may well provide the basis of a legal challenge that convinces your judge to declare the custodial arrest to have been illegally made.