The doctor has no direct authority over the company--he is not an executive or manager at it. Therefore, the company is not obligated to follow his instructions. If the company reasonably believes that you cannot do your job properly, and/or that there is some safety or liability risk (for example: you may re-hurt your foot or delay your recovery due to something that happens at work, and therefore have some claim against the employer) they have the right to not employ you until it appears that you can do the job or won't pose a liability risk. If their belief that the boot poses a significant problem is not a reasonable one, they may be (depending on the exact circumstances) committing illegal disability-based discrimination; but if the boot could reasonably poses a significant risk or problem, then what they are doing is most likely legal.