It was probably Bush 43, I doubt senior would drive drunk. There's also something called diplomatic immunity for visiting heads of state. It would not make sense for Canada to deny entry of a US President over something like which happened decades prior. Also when it happened (and I lived that era) a DUI was little more than a more expensive moving violation. They'd either drive you home or take downtown to sober up and then let you go.
I don't think Canada specifically targeted DUI, their rule is that any serious crime is disqualifying, and that's based on the seriousness of the crime in Canada. They consider DUI pretty serious (as does Europe and many other places). They do have a reasonable right to do that. We wouldn't let a convicted armed robber into the US.