Depends on how old the loans are, actually. Nowadays the federal government is the originator and servicer of all student loans. I am not sure if they bought older loans off the books of other originators or not.
Not sure how the details of the forgiveness aspect works for these cases, but I doubt the gov't is asking any servicers of government backed student loans to take a loss.
This option for forgiveness in the case of disablement was actually already available, but I guess it was a huge bureaucratic headache to actually get it done if you applied for it. Thus it was almost never used. Apparently what they have done is streamline the process to make it much easier, and send out notices to those eligible. Hardly an earth-shattering change.
Lastly, the government loans out money to students, and takes payments with interest in return. It does not cost the taxpayers a dime, as the government actually makes money on the loans it issues. In this case, the 'taxpayers' being burdened are the students actively paying their debt, as the interest could be construed as a tax going into federal coffers. So you have students paying back loans, and subsidizing the forgiveness of loans for some.
There's a broader point to be made about whether the debt should be forgiven based on how disabled someone actually is. Disability fraud is a big thing, and this might make it bigger. Also, I've met blind, deaf, wheelchair-bound, and one-armed people at all levels of academia. So there is at least some question as to whether your disability actually inhibits you from performing in your chosen career. Lose function of an arm and a leg due to a stroke? That concert pianist job you had is probably gone for good. But a computer programmer might be more or less unaffected. It would have to be on a case-by-case basis to get it right. But that more nuanced discussion is not what you brought up at all.