Making public colleges free isn't... free. The perception is often that public universities are already getting most of their money from the state or federal government, so why not just close the gap. But the typical funding level is between 40-50%. The remaining 50-60% is currently covered by tuition and philanthropic giving. So closing the gap to make universities free to the public would involve (in most cases) more than doubling the current level of state/federal funding.

That doesn't mean it should not be done - but the magnitude of the investment should be made clear.