Why do you think the TN is not a work visa? The US State Department website literally refers to it as "the TN visa", https://travel.state.gov/content/travel/en/us-visas/employme... :
> The nonimmigrant USMCA Professional (TN) visa allows eligible citizens of Canada and Mexico to work in the United States as USMCA professionals in prearranged professional level business activities for U.S. or foreign employers.