If you think it should not be free only to those on Pension Credit, how would you determine who should be exempt?
If it is based on income or savings, there will always be those who fall just outwith the maximum figure.
The same applies to means-tested benefits where entitlement is reduced according to income or capital. If someone has an income or capital that exceeds the limits by even a penny, that benefit is not payable.
Regardless of the method used, there will always be those who miss out.