I am arguing that if there is no clear danger one should not go overboard, proportionality is what matters.
The USA and Canada are to all intents and purposes one when it comes to this kind of control. There are some borders that have been constructed with a biological air-gap in mind but that's not one of them.
This is one of them:
https://en.wikipedia.org/wiki/Dari%C3%A9n_Gap
And it is effective because it is both deep and there are no easy ways around it.
I've visited places where there was definite risk of contamination and there are plenty of historic examples (of whole continents, for that matter, such as Australia) where eco-systems were upset by bringing in pests, diseases, new predators or animals without local predators so I'm well aware of the risk.
But border guards going overboard because of an apple makes zero sense to me, it is an over-reaction to something that is not a threat.
Yes, hypothetically you might be able to find a path through which a new disease might spread from one country to another because of a single apple. But since we allow animals to freely roam the Canadian/US border at will, since all the diseases that might affect your apple orchard as far as I'm aware are spread on both sides of the border since as long as we can remember in most cases and since that apple could have easily been inspected on carrying some kind of scary insect (which likely wasn't present or the owner would not have brought it) and everything else it might carry (some kind of blight) would likely not be carried past the point where it would survive normal indigestion and disposal the risk is so vanishingly small that it becomes a display of power rather than something useful.
Case in point: The many apples that I've carried from one side of the border to the other without so much as a batted eye. It's either consistent enforcement or you might as well not enforce at all and I highly doubt the US/Canadian border is so well established that it would not be possible to carry truckloads of apples across if one were so inclined.
Not all of the border is patrolled, even though you might believe other wise.
When the pig disease fear was in full swing the measures were laughably inefficient and inconsistent, and that was an actual threat with serious risk of contagion.
West Nile is another nice example: Mosquitoes don't see that imaginary line we call a border. Same with ant species, beetles and other big time destroyers of lives or crops.
Having the border patrol confiscate apples might sell a few more apples South of the border and it might give the official a boner but it isn't going to achieve much to keep a country safer, rather the opposite, it's going to cause people to find creative ways to get their stuff across the border with the chance that someone does smuggle a box with seed potatoes carrying blight across un-noticed.
And that would be a problem. An apple clearly intended for consumption is no more dangerous than any other item intended for consumption including sandwiches, liquids and so on.
If you're going to forbid apples you might as well forbid those otherwise it is inconsistent.