That is... Weird. I'm a Canadian who does business in US (but I do not reside or physically work there). I have always understood TN as the way to go. I have relatives on L-1 and P-1 visas and I always give the L-1 guy a hard time because the path to become a citizen is beholden to his (now bankrupt) employer. Never even considered that any of us would qualify for H1B. I suspect it is "lazy" legal departments who just file Canadians with all of their international people.
There is a whitelist of occupations that qualify for TN visas. Investment banking, for example, is not on that list so my i-banking friends had to get H1B's.
Thank you for this, I remember reading about this a few years ago. IIRC my work would be covered but I'll double check!