"Why, did their employment contracts state that they could not resign during an emergency?"
I have no idea what their employment contracts say.
I suspect that they did swear an oath to protect the people they served, but oaths are only as good as a person's word.
They can resign. They should just shouldn't be hired for another job in law enforcement or an emergency service.
When he took a job as a police officer he assumed the responsibility to be there when he's needed.
Any job requires some level of personal responsibility. Even a job flipping burgers requires you to be responsible enough to show up for work.
If you don't show up for work in a situation where you have important assigned duties, expect to be fired, and expect other employers not to want to hire you.
I agree.