Why not make college education free - with a payback?

Many decades ago I received "student loans," which, combined with small jobs, allowed me to pay my way through college and graduate school. I remember thinking at the time that it would have made more (financial) sense to both schools to allow me to go through for free in return for promising to give them perhaps 1% of all my future income. I would gladly have signed on the dotted line, videotaped an acknowledgement of my promise, and then would have foregone the library job, the summer job etc. If they were right that college and postgraduate work would eventually pay off well for me, then it should logically also pay off well for them (and it would have). But that wasn't an option at the time, and I had not heard of the concept ever being implemented by any universities or colleges.

Surprisingly, this past week I saw my concept put forth in a Slate editorial by Eliot Spitzer. He calls them "income-contingent" loans and says they have been "successful and commonplace in Europe and Australia." The concept has been advocated by Milton Friedman and James Tobin - "two Nobel laureates of decidedly differing worldviews."

I don't understand why this concept hasn't been advocated more forcefully or implemented by American universities.