Because the DUI laws aren't designed to protect people, they are designed to extract money out of citizens for the courts and their buddies providing 3rd party services. Someone blows exactly the limit that is within the error range of the breathalyzer? Still get charged just as hard for a DUI because that is literally thousands of dollars the court will receive. Oh sure if you got $10K to drop on a lawyer it will go away easily, but for anyone that has a public defender they are shit out of luck. Defending yourself in court with a public defender is just increasing the risk and liability because if they lose the case they now have to pay thousands of dollars more for court costs, which pushes people to taking shitty plea deals.

Oh sure there are plenty of people who are guilty and have a problem, they get caught too, but the courts want money so they aren't just going after the problem, they are charging any and every person possible. Some people get charged DUIs for annoying a cop or being tired, and even if their blood work comes up clean, do they drop the case? No. They just argue they were high on some other drug that they didn't test for.

> Someone blows exactly the limit that is within the error range of the breathalyzer?

I hate to say this, but how about... not drinking and driving? Drunk driving is a massive problem in the US and accounts for a good proportion of all driving fatalities. And your attitude sounds precisely like what causes this issue: unless the penalties are painful, people keep trying their luck in hopes of blowing "exactly the limit".