Not true. At-will is state by state. And, of course, there is nothing stopping either you or your employer from negotiating different terms.
Montana isn't at-will after 6-months of full-time employment and follows the French system.
The other 49 states are at-will employment, with generally the same rules with limited restrictions.
I stand by my original comment. Even Montana has at-will employment for the first 6 months.