They can't make you use your pto time when you get called off, but, on the other hand, they can put whatever qualifications/limits on providing health insurance they want ... Sounds like you're saying that they're telling you you have to use the pto
in order to meet the 72-hour requirement to get the paid health insurance, right? So, you can still take the unpaid time off
but you'd lose your insurance.
That, I believe, is perfectly legal for them to do (another thing that's wrong with our system ...

)