Can a company insist you be on call 24-hours per day? Can it fire you for personal commitments on your time?

I work for a media company and sometimes cover news during my time off. The editor insists that I am on call whenever she needs me, nights and weekends, but I am not staff; I am not salary and have been issued no phone or pager. I am simply an hourly employee at the bottom of the food chain. I told her I have to take a second job that is not with media, a competitor or in any way creates a conflict of interest. She threatened to fire me simply because it could mean I wouldn't be available to her at a whim outside of the regular work week. Is this legal?