They're only an income tax if you can't pay them off. Call it a tax on the poor and normal, while as ever the wealthier cohorts get a nice break. This is why there should have been an actual graduate tax.
That and the fact that a graduate tax would at least have been predictable, unlike, say, the soaring interest rates on student debt at the moment.
(Does raise the question of whether student loan repayments, which are also linked to income unlike other loans, and have been called a "graduate tax" by some of their advocates, are also an income tax that applies only to people born after a particular date ...)