Democratic Primaries

It's just that they'll pay for it with a delay once they have a job thanks to the education they received. It's an investment by the country in its citizens who will ideally pay back that goodwill.
You are right, some will pay for it who don't benefit directly from it. That could be said for pretty much all tax expenditures. With these kind programs, you can always point out a degree of unfairness when something is geared towards an advantage for the society as a whole that might never reach the individual.

I still think it's a good thing to make higher education accessible to as many as possible but the way it's done is certainly a matter where opinions differ.