Below ~$45k/yr, the actual income tax rate paid is zero or negative after filing for a return. These are the 47% of Americans that Mitt Romney correctly described as paying no income tax.

A $21/hr tax-free job minus driving costs is still equivalent to a $15/hr job. It's definitely not equivalent to a $21/hr job riding around in someone else's ambulance burning someone else's gas. And from what I'm seeing, residents make ~$60k/yr. That job is also a stepping stone to a much more lucrative profession.