Job asks me to do volunteer work before being paid?

The fuck?

While applying for a role, I get this question:

Would you be open to volunteering for 30 days before acquiring the position?

Is this common? Should I do it? Can't they just fire me after my volunteering stint has finished?

usa jobs
usa jobs resume
usa hotel jobs
usajobs
usa jobs federal government
usa job in ksa
usa jobs
usa jobs login
usa jobs gov
usajobs.gov
www.usajobs.gov
usajobs.com
usajobs
usajobs.gov official