The moral argument in favor of rejecting this app pivots around whether it does in fact enable more drunk driving. If it does, there doesn't seem to be good justification for allowing the app to exist. I don't think the calculus should be any more complicated than this. Edit: I'll disclaim that I have a bias, having had a friend die drinking and driving.
Someone who's salary I pay, using equipment I bought, forcibly stopping me on a street that I paid to build, and asking me what business I have being there.
Edit: What is the criminal penalty for having a .08 while performing law enforcement duties?