I'm not a lawyer, but IIRC, the Uniformed Services Employment and Reemployment act makes a specific exception to the requirement that a person on active duty must be reinstated if the employer can show that the employee would have been laid off, transferred or terminated had he/she not been called to active duty.

So no, Federal law is not going to make your company go broke by holding open jobs for Guardsmen when it really needs to lay people off.