You're talking about duplicate discussions, but if HN trims query parameters then how can it tell if a submission to a site that relies on query parameters is a duplicate? Every URL on such sites would become identical, making different pages appear to be duplicates. That would dramatically hurt the visibility of such sites, unless HN turned off the automatic duplicate prevention for those sites, in which case the situation would be worse than the current situation.
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.