Obamacare is quite recent, and Medicaid has only been available in all states since 1982. There are some tax deductions for medical care, though those usually only apply to middle-class workers with benefits anyways.

Which healthcare subsidy would result in riots in the streets?

Also keep in mind that the US has been without the 16th amendment longer than it has been with it, so anything based off of an income tax must not be intrinsically important enough to automatically result in street riots. Though of course an irresponsible and sudden withdrawal could certainly cause problems.