Law? I think you used the right word to describe that... a
myth.
Many companies (and Governments too) however DO have policies in place to allow Business class bookings for flights over "X hours"... It can (and does) vary...
I would imagine (as a cynic) that many American companies set that limit slightly longer than a TATL flight!
If your company has an "always Coach" policy I think you're out of luck looking for a legislation based solution!