The Canada-US border was not imposed by colonial Britain. It was a compromise between the US and Britain after the American Revolution and War of 1812.

In this context, the distinction between "the US" and "Britain" is entirely immaterial. People of European descent came, had some internal bickering, drew lines on maps, didn't care about the opinion of the people already living there.

By the time the US/Canada border was drawn, the majority of people "already living there" (90%) were of European descent.

He does have a point. The border was straight because the people living there mostly (European descendants, the locals were already disenfranchised by that point) didn't give a crap about the border.

I'm Romanian. Ask a Romanian what he'd think about straightening the border around Timisoara, thus giving it to Hungary. Many of those people would die rather than letting it happen. Those borders represent family possessions, cherished childhood memories, remnants of stories past that bond together people on either side of the border, etc.

Normal, i.e. organic borders, are messy and rarely straight.

Yes, because native populations had been decimated by disease.

And that war started after the native inhabitants had been almost driven into extinction.