My understanding on the dui thing (or any other crime here in the US that was considered more serious in Canada, such as a misdemeanor here that was felony there like dui) was that if you didn't make advance arrangements you would be turned away at the border, you could fairly easily do this through a lawyer and paying of course. No personal experience but do believe someone I know was turned away, but it's been many years and I don't remember who that was. Maybe it's not the best information but I had looked into this at one point, while passenger/personal crossings aren't my old trade, I'm a former US Customs broker and was asked about this and from what I looked into with my agents in Canada, had verification.
I definitely had one friend who was turned away several years back for a theft as a minor that wasn't even supposed to be on the books any more (but computer still had it); his buddies dumped him and his bike at the border and he came here instead of Whistler (we also have good mountain biking) and I knew he was supposed to be in Whistler and we had a nice conversation about it.
Was the waiver offered the whole time as you passed thru Customs since what 2002, I think it was around then when it went into effect? Customs officers often have much leeway in what they choose to enforce, too.
Both Homeland Security and Canada Border Agency have access to each other's national crime databases, and if you attempt to cross the border, that data will be accessed and form part of a permanent record on you. If that record is later expunged or removed, that won't be reflected in the information the border agents have on you ... in hoth countries policy is to never delete data, period.
So (this is just an example) let's say you were caught smoking pot in Canada, with a partly smoked joint in your hand when arrested. That is enough to lay a charge; in fact pipe residue is enough to lay a charge in Canada. So you go before the judge, you are a first time offender, he gives you an Absolute Discharge, which is an option for non-indictable offfenses in Canada (the Crown has the option of proceeding by Summary Conviction or Indictment, which requires a Preliminary Hearing).
Note that in Canada every offense is a Criminal Offense ... there is no such thing as a misdeanor theft, for example. In the US, all Criminal Code convictions are considered "equivalent to felony" as far as deciding admissibility to the US.So shoplifting is a felony conviction to Homeland Security.
Later that year, you attempt to cross the border into the US, you answer "No' when asked if you have a Criminal Record, as this is often suggested, even by Lawyers who should know better, that an Absolute Discharge means you have no Criminal Record. Let me state right here that you do in fact have a Criminal Record if you have been in court for any reason, including if you were acquitted or the charges dropped before you entered court. Your record will exist until you die or reach the age of 95. * Also, Lying to a Border Agent is grounds for refusal, and you just lied to him, as he can see right there on the screen you were convicted of possession, and that you were refused will be part of the permanent record at the border in either Country.
What an Absolute Discharge really means is your record of conviction will be kept in a separate, un-searchable area in the Criminal Records data repository in Ottawa. It can be moved back into the searchable active area if you commit further crimes. It also takes time to do the record management.
That last statement is where our fictional pot smoker gets in to trouble. He attempted to cross the broder into the US before enough time had passed to move his Criminal Record to the non-searchable area. So the Border Agent accessed CPIC (Canada Personal Identification Computer) record that every officer in Canada can access in the car or by radio transmission ** if on foot. And CPIC reports back that he was convicted of possession of marijuana in Canada six months earlier. From that point on, that information will be in all US Homeland Security databases, affecting Air Travel across the US even if no stop on US soil, and travel to the US, where a drug conviction of any kind makes you inadmissible.
* The Police Agency that layed the charge also has records of the event. They are never required to delete any data either, so if you were busted in Toronto and your record is not in the CPIC database because you were given an Absolute Discharge and enough time ... typically two years ... has passed since the conviction, when you are stopped in Toronto they will have a complete record of the event, which will be limited to you being charged and what you were charged with.
Because the courts are a different branch, they won't have a record of whether you were convicted or acquitted, but they will know they are dealing with a pot smoker who was given an Absolute Discharge ("get out of here, kid") or a Conditional Discharge (probation, pay a fine, anything except even one day in jail),because the record back from Toronto PD headquarters will be "charged, no conviction".
There is only one way to get a "charged, no conviction" reply, and that is if you were given a Discharge. The CD is distinct from a sentence of probation, a fine, etc so don't confuse the two. The latter means a permanent searchable Criminal Record.
If an Absolute Discharge was the result of your court appearance, they will be prohibited from offering that information to other law enforcement agencies, so if you are stopped in Hamilton they cannot access Toronto's information. But ... you sometimes hear of officers investigating a crime traveling to another jurisdiction as part of their inquiries. What records do you suppose the officer is looking at?
** Canada's CPIC and the US's FBI records databases have been shared since the 1970's. Sometime around the late 1980's both countries got around to making it legal by passing legislation to that effect. Because of the numerous police agencies in the US, some local police used to have limited access in the car, State Police have always had access as have RCMP and all local police in Canada. These days even small American local forces have instant access. It takes about 30 seconds to get a full report that includes data from both countries.