The fact that nobody expects to be detained for the equivalent of a DMV trip is a good indication that this was "unreasonable" per the plain meaning of the word. Maybe case law says otherwise. But ask a "reasonable" person on the street what the appropriate response of a customs agent to someone (citizen or not) who forgot their ID to take care of some bureaucratic hurdle on a package is, especially when the agency was supposed to be aware of the person's arrival (note the mention of the email in the article). That's what I care about.
I should mention, by the way, that the pedestrian lane at a US-Canada border crossing typically has a queue on the Canada side, and the border agent calls groups in one-by-one, just as at an airport. I have no idea how someone just trying to interact with customs, who drove up and parked their car on the US side, could be confused with someone trying to enter the US. It's worth looking at a map of the Peace Bridge [1]. This isn't a checkpoint some ways away from the border; it's a road with one way into the US, through the checkpoint (and the Niagara River in between the two countries!) Unless you have a flying car, there is no way you could end up with a car on the US side without having already gone through border security. The incompetence itself is ridiculous.
No they wouldn't. In many cases it's impossible to put a border checkpoint at the literal border. That's why they're often at choke points in the transportation system near borders.
I think there's a reasonable debate to be had about whether those choke points should be on highways dozens of miles from the literal border. That's not the situation here.
> In fact, I suspect if you were to poll Americans, most probably think that this is already the case. That's why this was an issue to begin with: presumably if the man had known he was going to be detained if he didn't bring his ID, he would have done that.
How people expect border crossings to work has no relevance. The question is what is an "unreasonable" search and seizure under the fourth amendment. The Constitution uses the fuzzy term "unreasonable" precisely because it acknowledges the legitimate need of the government to search and detain people without a warrant. Effectuating the nation's sovereign right to control its borders is one of those reasons. A rigid mechanical rule that only permits searches of people as they cross the literal border cannot be reconciled with the much more--well, reasonable--language the fourth amendment uses.