You're paying a tiny fraction of someone else's student loan because the person concerned is very severely disabled and has highly limited prospects in the labour market. As /u/WholeGrainBeetus point out in some cases it is unrealistic to expect a person who can only perform restricted types of work to have the same earning and repayment capacity as an able-bodied person.  I agree with /u/Carsandsarcasm that disabled people should be encouraged to participate as much as is possible but we also have to recognise that disabled people face significant barriers and removing some of the challenges we can tackle easily makes a lot of sense.