If They are being paid, then taxing the income makes sense.
They make it sound as if unpaid volunteers, who are not being paid, would be billed state income tax.
The way I read it is that if you are getting some kind of regular check from a business or government entity in (let’s say) Kentucky, if you are in fact volunteering in NY, Cuomo would see your Kentucky income as partially his, because you earned it in KY while you were in NY.