Is the Single Responsibility Principle applicable to functions?

According to Robert C. Martin, the SRP states that:

There should never be more than one reason for a class to change.

However, in his book Clean Code, chapter 3: Functions, he shows the following block of code:

    public Money calculatePay(Employee e) throws InvalidEmployeeType {
        switch (e.type) {
            case COMMISSIONED:
                return calculateCommissionedPay(e);
            case HOURLY:
                return calculateHourlyPay(e);
            case SALARIED:
                return calculateSalariedPay(e);
            default:
                throw new InvalidEmployeeType(e.type);
        }
    }

And then states:

There are several problems with this function. First, it’s large, and when new employee types are added, it will grow. Second, it very clearly does more than one thing. Third, it violates the Single Responsibility Principle (SRP) because there is more than one reason for it to change. [emphasis mine]

Firstly, I thought the SRP was defined for classes, but it turns out it's also applicable to functions. Secondly, how is that this function has more than one reason to change? I can only see it changing because of a change on Employee.