Obviously few people subscribe to several newspapers, but I expect HN users subscribe to one or two.
I don't know the official reason, but this seems very obvious to me. Because there frequently are paywalled articles that are of value to the HN audience.
archive.ph
The fact that you have ways to avoid paywalls doesn't change my question imho.
Wouldn't it be more constructive to give arguments first for the opposite: Why should hacker news limit paywalled articles. Since you acknowledge you are already offtopic, you could add a little more than just ask a question that is a little too broad for an easy answer.
I'd say HN should prefer the URL that fits its readers best. Which in this case means paywalled URLs should be replaced with the available workarounds for that problem, ie URLs through that archive.ph.