The government already does compel companies to perform work. Every other thing Apple already did to comply with the order was work. You think it was free? There are myriad examples of companies being compelled to do work to help an investigation.

If Apple could make the case that the change would be so burdensome that they can't feasibly do it without harming the company, they have a case to resist. I don't believe it, but they have to make that case.