Why would they do that? If they are providing it free to those at high risk, why not provide it free to those at low risk? Why would they charge for this partic

Why would they do that? If they are providing it free to those at high risk, why not provide it free to those at low risk? Why would they charge for this particular treatment when all other treatments are free? How could they ensure those who are poor and cannot afford to pay still get vaccinated?