This doesn't sound so good on the surface but the fact is poor people pay way less taxes than middle and upper class people so the brunt would be payed by high earners or college graduates anyway. Under Bernie policies free college would go hand in hand with a 15 dollar an hour minimum wage (a raise for the poorest) and Universal Health Care, (a raise for all but the wealthy). When you factor in most college graduates making more money and paying more taxes over time this is probably a non issue.
That does nothing to address the social benefits of getting a college education because smarter more educated people do more, have more, live longer, and give more back than others. In the end this is good policy idea that would raise the whole country up IMO.
I would not be against making college way less expensive rather than free as an alternate option.