What law does this kind of app aid a person in breaking?
I do not jest when I ask this, because it is NOT illegal to avoid contact with the police. Driving is not a right, but rather a privilege. However, you DO have the right to travel from one place to another free of undue and unwarranted harassment. Because of the way that DUI checkpoints are conducted, they absolutely qualify as undue and unwarranted harassment. You personally may not mind being the presumption of guilt that hovers over you at a DUI checkpoint, but most reasonable people resent the mindset of police who are looking for any excuse to slap you with a ticket--or worse. I consider the ability to avoid unnecessary interaction with an agency that does not have my best interests in mind to be a legitimate use. If you don't, then you really need to get a clue.
Furthermore, the locations of DUI checkpoints are published beforehand. Would you also take newspapers to task for publishing this information? You could certainly use it to get plastered and then avoid the cops. The checkpoint locations are intended to be public knowledge, and trying to restrict that knowledge is not too good an idea.
Yes, it is illegal to drive when drunk. Here's the thing, though--as soon as you get behind the wheel of a car you can't control (for nearly any reason) and start driving it, you've already broken the law. Once you're truly drunk, you don't have the mental capacity to take a route home based on where the police aren't. If you can think ahead and plan out your trip home based on a DUI checkpoint alert program and actually stick to it, then you probably aren't the danger to the driving public that MADD and the police say you are.
I haven't even gone into the inaccuracy of breathalyzer readings, nor the fact that field tests are designed to be failed. I could, but I trust that I've made my point.