Paywalls for knowledge should ethically be forbidden.

according to Kants categorical imperative: one maxim may be "all knowledge should be availabe freely" which would mean that all libraries and other educational institution would have to be completely free as well. While the access to public libraries is very cheap (in Germany) there would still be

Pragmatic Fallacy

The argument appeals to hypothetical implementation and infrastructure costs as a refutation rather than, on its face, arguing within the context of Kant's formulation (or otherwise) , why open flow of information is (un)ethical.