abc the short answer is: This is just the way it works.
Awhile back I tried to solve this issue by adding a whole bunch of invisible marks over in the Pacific to try to trick Tableau into thinking the center of my marks where North America. I never really got anything acceptable. Essentially T takes the average of all your longitudes and makes that the center of your map. (At least that's what I think T is doing.) And there is no way to override this. (And if I remember correctly you can't even 'pan' this issue away.)