according to rawls second principle of justice the poor deserve to get the most benefit out of a social inequality. Since Paywalls dont allow all members of society to access knowledge in the same way, they do not benefit the poorest people the most.
Forbidding partly what is allowed to do with intellectual property requires at least partly a very strong left-wing course in politics. A liberal party would never do so because that would raise the question why they don't do the same with water for example.
There are very many open journals, databases, pre-print services and Wikipedia-like communities. Subscription journals may still require payment to publish, do not pay reviewers and/or charge for the journal or articles, so they can simultaneously take resources from writers, reviewers and readers.
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
massive additional costs for the government to cover and probably additional infrastructural costs as well.
According to this maxim there also could not be any private universities anymore. All in all such a maxim would need our current system to change in a drastic way that seems very far-fetched for now.
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.