You are operating under the rather naive assumption that the people and resources used in fusion research are fungible with the people and resources used in other things.
When you have a bunch of people who know how to build nuclear bombs sitting around with nothing to do, you damn well keep them busy before another country finds them a job.
When you have a bunch of people who know how to build nuclear bombs sitting around with nothing to do, you damn well keep them busy before another country finds them a job.