When I lived in Germany, there were two general classifications of DUI (this is a very rough generalization). If you came in between .05-.079, your DUI was considered more of a traffic infraction. If it was above .08, it was treated more like a criminal offense.

I'm not sure what the Canadian authorities consider to be DUI, but I can't help but think that if they differentiated between the serious DUIs and the less serious DUIs, they would not have to turn around so many people. Granted, drinking and driving isn't a good idea regardless, but there is a difference between driving after a beer or two and driving while completely inebriated.

Maybe another workable condition of entry is to forbid anyone who has a prior DUI (or a recent DUI) from driving while in Canada under the penalty of permanent expulsion if they are caught driving in Canada. It just doesn't make a whole lot of sense to keep people out who may have made a mistake earlier in their life, or people who can demonstrate that they won't drive while in Canada (for instance, someone who is entering with another person who can drive or someone who has train/air reservations).