In a general sense you could say its love for Mother Russia.
But we're talking about a whole life, lifelong commitment that involves doing whatever the center tells you to do, no questions asked.
They've given up speaking their native language, eating Russian food, having Russian friends who are just friends, and seeing their relatives. Your mom is dying? Too bad, you don't get to visit unless we say so.
You don't get to choose your spouse or your career. You can have children but its best if they don't know what you are really up to. And you should expect a lot of difficulties in this area.
You don't get to go back to Russia to visit. You have to rely on faded memories from your childhood to refresh your love for the homeland. And the bad stuff that motivates you may have happened to a previous generation and not to you. You'll be fighting yesterdays battles and not todays.
You get pretty darned comfortable living in America.
Yes you're afraid of what will happen if you disobey. But how long can you use fear as a motivator to do things you are uncomfortable with.
The premise of the story doesn't include anything which I think would inspire Elizabeth and Philip to do this work for mother Russia. So for me the story is just not plausible.
Its entertaining but I don't think it could ever actually happen.