Why is this an issue?

The difference between private and protected visibility is that child classes can see and use protected members, but they cannot see private ones. Since a sealed class cannot have children, marking its members protected is confusingly pointless.

Noncompliant code example

public sealed class MySealedClass
{
    protected string name = "Fred";  // Noncompliant
    protected void SetName(string name) // Noncompliant
    {
        // ...
    }
}

Compliant solution

public sealed class MySealedClass
{
    private string name = "Fred";
    public void SetName(string name)
    {
        // ...
    }
}