> HN exists to share articles
Not to argue one way or another for that claim, but assuming it were true then enforcing a no query param constraint would allow greater visibility for the shared content as again it would prevent the specified case(s) of allowing it to get buried by submissions duplicating the link. Also, it seems pointless to go against the REST standard of using path params to identify a specific resource or resources while using query parameters to sort/filter those resources; disallowing direct linking to a filter does not make direct links to articles impossible. It is true that HN itself relies on query parameters to link resources that goes against the stated REST standard.
The second half of your comment is again on the wrong side of the is/ought divide. Websites that (a) have content worth sharing and (b) use query parameters to identify a specific resource do in fact exist. The question is not whether those websites should be doing that, it is whether HN should make it impossible to link to specific resources on those websites or not.