Well when you sign the employee handbook you agree to the fact that they technically could have issued a corrective action if you didn’t find your own coverage since it wasn’t 21 days in advance. And they can legally separate you if your new availability doesn’t fit the needs of the business. Considering that, it’s actually nice of them to cover the shift for you. They don’t actually have to unless it’s an emergency.

But that’s just the technical side of it.