Emergency healthcare is free. Cancer treatment might be free, but good luck seeing the specialists needed to get that treatment without some very explicit charity.
Medicaid is (1) not available to anywhere close to all poor people in the US (though it would have been for a particular poverty definition had ACA expansion been universal), and (2) doesn't provide free care to all the poor people to whom it is available (both premiums and cost sharing—effectively, deductibles—are allowed in State Medicaid plans), and (3) very often does have limits on available services, so while Medicaid exists, it does not meet the description made of healthcare being free in the US if you are poor without treatment restrictions that has been made.
What specific program are you talking about? Because you're wrong when you state this as an unqualified generality about poor people in the US, but identifying the exact error requires knowing what program you think is involved.
> There are no restrictions on treatment.
There is no program for low income healthcare for which this is true.