Petitioner = your employer

You have no choice (any argument) until your 485 is filed for 180 days. They did not care the H1b approval because you could go anywhere with a new H1b sponsor. But for 140 you need to stick with it for a while becaue you need THIS 140. That's why. Your employer is not an idiot and at meantime you need to behave good to them because they can cancel 140 at any time.