If public safety is their concern, like showstoppa said, there is so much out of control reckless driving that goes on, they could be pulling over people all day long. In most big cities, it's hard not to see something extremely fucked up happen every 10 minutes it seems like.
I would like to see how many drunk drivers get away scot free for refusing to cooperate with the standard tests. I bet the number is low to zero.
DUI is just a hot button issue. The amount of accidents each year by people falling asleep at the wheel, dwarfs the amount caused by DUI. I think a lot of it is the "moral" issue of drinking at all, that is the basis of this, but we could go on forever about that.
No, it's not ok to drink and drive, but it's not ok to do 100mph on an expressway, be so tired you fall asleep at the wheel, etc.... but people are guilty of those every day with little to no punishment. They make a choice to drive will tired, so what is the difference? They are still endangering other people.