No, it's not.
It flows out of the American doctrine of "employment at will". That means the employer can fire you at any time for any or no reason at all, and you can quit your job at any time for any or no reason at all. Every state except Montana is employment at will.
Given that they can fire you for no reason at all, it logically follows that they can fire you for refusing to let them look at your personal phone.
Now, over the years governments have constrained this. For example, you have the federal civil rights laws that say you can't be adversely acted against because of your race, gender, religion, national origin, etc. States and cities can (and have) gone further, adding things like sexual orientation, gender identity, etc.
Some states go further and say you can't be fired for your appearance. Or because of various lifestyle choices.
And I wouldn't be surprised if there are some states that do say an employer can't look at a personal phone unless the employer has reasonable grounds to suspect certain business-related things.
But the default would be that they can.